p    i 

J    7 


' 


COLLOID  CHEMISTRY 

AN  INTRODUCTION,  WITH   SOME 
PRACTICAL  APPLICATIONS 


BY 
JEROME  ALEXANDER,  M.Sc., 

Chairman,  Special  Committee  on  Colloids,  Division  of  Chemistry 

and  Chemical  Technology,  National  Research  Council 

Member  Amer.  Institute  Chemical  Engineers 


ILLUSTRATED 


NEW  YORK 

D.    VAN   NOSTRAND    COMPANY 

25   PARK  PLACE 

1919 


J>> 


COPYRIGHT,  .1919, 

BY 
D,  VAN  NOSTRAND  COMPANY 


Stanbope 

F.    H.  GILSON  COMPANY 
BOSTON,  U.S.A. 


PREFACE 

THIS  little  book  is  the  result  of  an  attempt 
to  compress  within  a  very  limited  space,  the 
most  important  general  properties  of  colloids, 
and  some  of  the  practical  applications  of  col- 
loid chemistry.  Its  object  will  be  accomplished 
if  it  is  helpful  in  extending  the  sphere  of  in- 
terest in  this  fascinating  twilight  zone  between 
physics  and  chemistry. 

J.  A. 

NEW  YORK, 
Nov.  1,  1918. 


415477 


TABLE   OF  CONTENTS 


CHAPTEB  PAGE 

I.  INTRODUCTION 1 

II.  CLASSIFICATION  OF  COLLOIDS 10 

III.  CONSEQUENCES  OF  SUBDIVISION 14 

IV.  THE  ULTRAMICBOSCOPE 17 

V.  GENERAL  PROPERTIES  OF  COLLOIDS 24 

VI.  PRACTICAL  APPLICATIONS  OF  COLLOID  CHEMISTRY  .  36 

BIBLIOGRAPHY 85 

AUTHOR  INDEX 87 

SUBJECT  INDEX 89 


COLLOID  CHEMISTEY 


CHAPTER  I 
Introduction 

Although  many  facts  and  principles  con- 
cerning colloids  have  from  time  immemorial 
been  known  and  utilized  empirically,  the 
scientific  foundation  of  modern  colloid  chemis- 
try was  laid  by  an  Englishman,  Thomas  Gra- 
ham, F.R.S.,  Master  of  the  Mint.  In  two 
basic  papers  on  this  subject,  the  first  entitled 
"Liquid  Diffusion  Applied  to  Analysis,"  read 
before  the  Royal  Society  of  London,  June  13, 
1861,  the  second  entitled  "On  the  Properties 
of  Colloidal  Silicic  Acid  and  other  Analogous 
Colloidal  Substances,"  published  in  the  Pro- 
ceedings of  the  Royal  Society,  June  16,  1864, 
Graham  pointed  out  the  essential  facts  regard- 
ing colloids  and  the  colloidal  condition,  and 
established  much  of  the  nomenclature  in  use 

at   the  present   day.     In   the  first   of   these 

i 


2   \  COLLOID  CHEMISTRY 

papers  Graham  says:  "The  property  of  vola- 
tility, possessed  in  various  degrees  by  so  many 
substances,  affords  invaluable  means  of  separa- 
tion, as  is  seen  in  the  ever-recurring  processes 
of  evaporation  and  distillation.  So  similar  in 
character  to  volatility  is  the  diffusive  power 
possessed  by  all  liquid  substances,  that  we 
may  fairly  reckon  upon  a  class  of  analogous 
analytical  resources  to  arise  from  it.  The 
range  also  in  the  degree  of  diffusive  mobility 
exhibited  by  different  substances  appears  to  be 
as  wide  as  the  scale  of  vapor  tensions.  Thus 
hydrate  of  potash  may  be  said  to  possess  double 
the  velocity  of  diffusion  of  sulphate  of  potash, 
and  sulphate  jti  potash  again  double  the 
velocity  of  sugar,  alcohol  and  sulphate  of 
magnesia.  But  the  substances  named  belong 
all,  as  regards  diffusion,  to  the  more  "vola- 
tile "  class.  The  comparatively  "fixed  "  class, 
as  regards  diffusion,  is  represented  by  a  differ- 
ent order  of  chemical  substances,  marked  out 
by  the  absence  of  the  power  to  crystallize, 
which  are  slow  in  the  extreme.  Among  the 
latter  are  hydrated  silicic  acid,  hydrated  alu- 
mina and  other  metallic  peroxids  of  the 
aluminous  class,  when  they  exist  in  the  soluble 


INTRODUCTION  3 

form;  with  starch,  dextrin  and  the  gums, 
caramel,  tannin,  albumen,  gelatin,  vegetable 
and  animal  extractive  matters.  Low  diffusi- 
bility  is  noj^the  only  property  which  the  bodies 
last  enumerated  possess  in  common.  They  are 
distinguished  by  the  gelatinous  character  of 
their  hydrates.  Although  often  largely  soluble 
in  water,  they  are  held  in  solution  by  a  most 
feeble  force.  They  appear  singularly  inert  in 
the  capacity  of  acids  and  bases,  and  in  all  the 
ordinary  chemical  relations.  But,  on  the  other 
hand,  their  peculiar  physical  aggregation  with 
the  chemical  indifference  referred  to  appears 
to  be  required  in  substances  that  can  intervene 
in  the  organic  processes  of  life.  The  plastic 
elements  of  the  animal  body  are  found  in  this 
class.  As  gelatin  appears  to  be  its  type,  it  is 
proposed  to  designate  -substances  of  this  class 
as  colloids,  and  to  speak  of  their  peculiar  form 
of  aggregation  as  the  colloidal  condition  of 
matter.  Opposed  to  the  colloidal  is  the  crys- 
talline condition.  Substances  affecting  the 
latter  form  will  be  classed  as  crystalloids.  The 
distinction  is  no  doubt  one  of  intimate  molec- 
ular constitution. 

"  Although  chemically  inert  in  the  ordinary 


4  COLLOID  CHEMISTRY 

sense,  colloids  possess  a  compensating  activity 
of  their  own,  arising  out  of  their  physical 
properties.  While  the  rigidity  of  the  crystal- 
line structure  shuts  out  external  impressions, 
the  softness  of  the  gelatinous  colloid  partakes 
of  fluidity,  and  enables  the  colloid  to  become 
a  medium  for  liquid  diffusion,  like  water  itself. 
The  same  penetrability  appears  to  take  the 
form  of  cementation  in  such  colloids  as  can 
exist  at  high  temperature.  Hence  a  wide 
sensibility  on  the  part  of  colloids  to  external 
agents.  Another  and  eminently  character- 
istic quality  of  colloids  is  their  mutability. 
Their  existence  is  a  continued  metastasis.  A 
colloid  may  be  compared  in  this  respect  to 
water,  while  existing  liquid  at  a  temperature 
under  its  usual  freezing-point,  or  to  a  super- 
saturated saline  solution.  Fluid  colloids  ap- 
pear to  have  always  a  pectous  modification; 
and  they  often  pass  under  the  slightest 
influences  from  the  first  to  the  second  condi- 
tion. The  solution  of  hydrated  silicic  acid, 
for  instance,  is  easily  obtained  in  a  state  of 
purity,  but  it  cannot  be  preserved.  It  may 
remain  fluid  for  days  or  weeks  in  a  sealed  tube, 
but  is  sure  to  gelatinize  and  become  insoluble 


INTRODUCTION  5 

at  last.  Nor  does  the  change  of  this  colloid 
appear  to  stop  at  that  point.  For  the  mineral 
forms  of  silicic  acid  deposited  from  water,  such 
as  flint,  are  often  found  to  have  passed,  during 
the  geological  ages  of  their  existence,  from  the 
vitreous  or  colloidal  into  the  crystalline  con- 
dition. (H.  Rose.)  The  colloidal  is,  in  fact, 
a  dynamical  state  of  matter,  the  crystalloidal 
being  the  statical  condition.  The  colloid 
possesses  Energia.  It  may  be  looked  upon 
as  the  probable  primary  source  of  the  force 
appearing  in  the  phenomena  of  vitality.  To 
the  gradual  manner  in  which  colloidal  changes 
take  place  (for  they  always  demand  time  as  an 
element)  may  the  characteristic  protraction  of 
chemico-organic  changes  also  be  referred.  .  .  . 

"It  may  perhaps  be  allowed  to  me  to  apply 
the  convenient  term  dialysis  to  the  method  of 
separation  by  diffusion  through  a  septum  of 
gelatinous  matter.  The  most  suitable  of  all 
substances  for  the  dialytic  septum  appears  to 
be  the  commercial  material  known  as  vegetable 
parchment,  or  parchment-paper.  .  .  ." 

At  the  beginning  of  the  second  paper  above 
referred  to,  Graham  states:  "The  prevalent 
notions  respecting  solubility  have  been  de- 


6  COLLOID  CHEMISTRY 

rived  chiefly  from  observations  on  crystalline 
salts,  and  are  very  imperfectly  applicable  to 
the  class  of  colloidal  solutions."  From  this 
it  may  be  seen  that  Graham  appreciated  the 
fact  that  all  the  laws  of  crystalloidal  solutions 
could  not  be  applied  to  colloidal  solutions. 
In  the  case  of  crystalloidal  solutions  the  dis- 
solved substance  is  present  in  a  state  of  molec- 
ular subdivision,  and,  according  to  the  ioniza- 
tion  theory,  is  in  many  cases  dissociated  into 
ions.  With  colloidal  solutions,  on  the  other 
hand,  we  have  a  lesser  degree  of  subdivision, 
and  the  particles  in  solution  are  larger  and 
more  cumbersome.  As  Graham  remarked, 
"The  inquiry  suggests  itself  whether  the 
colloid  molecule  may  not  be  constituted  by  the 
grouping  together  of  a  number  of  smaller 
crystalloid  molecules,  and  whether  the  basis 
of  colloidality  may  not  really  be  this  composite 
character  of  the  molecule."  This  is  to-day 
the  idea  generally  accepted. 

COLLOID  CHEMISTRY  DEFINED 

Colloid  chemistry  deals  with  the  behavior 
and  properties  of  matter  in  the  colloidal  con- 
dition, which,  as  we  now  know,  means  a  certain 


INTRODUCTION  7 

very  fine  state  of  subdivision.  While  there 
are  no  sharp  limitations  to  the  size  particles  in 
colloidal  solutions,  it  may  in  a  general  way  be 
stated  that  their  sphere  begins  with  dimensions 
somewhat  smaller  than  a  wave  length  of  light, 
and  extends  downward  well  into  dimensions 
which  theory  ascribes  to  the  molecules  of 
crystalloids.  (See  Table  II,  p.  12.) 

SUSPENSION  vs.  SOLUTION 

With  the  aid  of  the  ultramicroscope,  which 
renders  visible  particles  approaching  in  mi- 
nuteness molecular  dimensions,  Zsigmondy  has 
shown  that  there  is  no  sharp  line  of  demarca- 
tion between  suspensions  and  solutions,  but 
that  with  increasing  fineness  in  the  subdivision 
of  the  dissolved  substance,  there  is  a  progres- 
sive change  in  the  properties  of  the  resulting 
fluids,  the  influence  of  gravity  gradually 
yielding  to  that  of  the  electric  charge  of 
particles,  of  surface  tension  and  of  other  forms 
of  energy.  Thus  in  the  case  of  metallic  gold, 
subdivisions  whose  particles  are  1  M  and  over 
act  as  real  suspensions  and  deposit  their  gold, 
whereas  much  finer  subdivisions  (60  AM  and 
under)  exhibit  all  the  properties  of  metal 


8  COLLOID  CHEMISTRY 

hydrosols  or  colloidal  solutions.  In  the  ultra- 
microscope  the  coarser  subdivisions  show  the 
well-known  Brownian  movement,  which 
greatly  increases  as  the  particles  become- 
smaller,  until  at  the  present  limit  of  ultra- 
microscopic  visibility  (about  5  MM)  it  becomes 
enormous  both  in  speed  and  amplitude. 

On  the  other  hand,  there  is  no  sharp  dis- 
tinction between  colloids  and  crystalloids,  but 
as  the  particles  in  solution  become  smaller  and 
smaller,  the  optical  heterogeneity  decreases 
correspondingly,  finally  vanishing  as  molec- 
ular dimensions  are  approached.*  That  even 
crystalloid  solutions  are  not  in  a  strict  sense 
homogeneous,  is  indicated  by  an  experiment 
of  van  Calcar  and  Lobry  de  Bruyn  (Rec.  Trav. 

*  In  an  article  entitled  "Pedetic  Motion  in  Relation  to 
Colloidal  Solutions  "  published  in  Chemical  News,  1892,  Vol. 
65,  p.  90,  William  Ramsay,  Ph.D.,  F.R.S,  (afterward  Sir 
William  Ramsay),  clearly  expressed  this  view  in  the  fol- 
lowing words:  "I  am  disposed  to  conclude  that  solution 
is  nothing  but  subdivision  and  admixture,  owing  to  attrac- 
tions between  solvent  and  dissolved  substance  accompanied 
by  pedetic  motion;  that  the  true  osmotic  pressure  has, 
probably,  never  been  measured;  and  that  a  continuous  pas- 
sage can  be  traced  between  visible  particles  in  suspension 
and  matter  in  solution;  that,  in  the  words  of  the  old  adage, 
Natura  nihil  Jti  per  sattum." 


INTRODUCTION  9 

chim.  Pays-Bas,  1904,  23,  218),  who  caused 
the  crystallization  of  a  considerable  part  of 
saturated  crystalloid  solutions  at  the  periph- 
ery of  a  rapidly  rotating  centrifuge. 


CHAPTER  II 
Classification  of  Colloids 

The  broadest  classification  of  colloids  is  that 
of  Wolfgang  Ostwald  (Koll.  Zeitschr.,  Vol.  1, 
page  291),  who  grouped  them  according  to  the 
physical  state  (gaseous,  liquid  or  solid)  of  the 
subdivided  substance  (dispersed  phase),  and 
of  the  medium  in  which  the  particles  of  the 
subdivided  substance  are  distributed  (disper- 
sion medium).*  Table  I  (page  11)  shows  the 
nine  resulting  groups  and  gives  some  instances 
of  each. 

Ostwald's  classification,  however,  is  more 
theoretical  than  practical,  for  the  properties  of 
colloids  are  dependent  mainly  upon  the  specific 
nature  of  the  dispersed  substance  and  its  degree 
of  subdivision.  Following  Hardy,  Zsigmondy 
divided  colloids  into  two  classes,  the  reversible 
and  irreversible;  the  former  redissolve  after 

*  G.  Bredig  proposed  to  call  colloids  "microheterogeneous 
systems."  W.  Ostwald  called  them  "dispersed  heterogeneous 
systems,"  which  expression  was  contracted  by  P.  P.  von 
Weimarn  into  the  term  "dispersoids." 

10 


CLASSIFICATION  OF  COLLOIDS 


11 


desiccation  at  ordinary  temperatures,  whereas 
the  latter  do  not. 


TABLE  I 


Dispersed 
phase. 

Dispersion 
medium. 

Example. 

Gas  

Gas  

No  example,  since  gases  are  miscible  in  all 

proportions. 

Gas        

Liquid  

Fine  foam,  gas  in  beer. 

Gas     

Solid  

Gaseous     inclusions    in    minerals     (meer- 

schaum, pumice),  hydrogen  in  iron,  oxy- 

gen in  silver. 

Liquid  

Gas  

Atmospheric  fog,  clouds,  gases  at  critical 

state. 

Liauid 

Liquid  

Emulsions  of  oil  in  water,  cream,  colloidal 

.LJ114  UJ.U.  .    ...... 

water  in  chloroform. 

Liquid  

Solid  

Mercury  in  ointments,  water  in  paraffin  wax, 

liquid  inclusions  in  minerals. 

Solid    

Gas  

Cosmic   dust,   smoke,   condensing   vapors, 

(ammonium  chlorid). 

Solid  

Liquid  

Colloidal  gold,  colloidal  sodium  chlorid,  col- 

loidal ice  in  chloroform. 

Solid  

Solid  

Solid  solutions,  colloidal  gold  in  ruby  glass, 

coloring  matter  in  gems. 

Table  II,  taken  from  Zsigmondy,*  illustrates 
this  classification,  and  shows  how  colloids  hav- 
ing the  same  particle  size  or  degree  of  sub- 
division may  nevertheless  act  quite  differently 
because  of  specific  differences  in  the  nature  of 
the  dispersed  substances. 

*  Colloids  and  the  Ultramicroscope,  J.  Wiley  &  Son,  Inc. 
(Translation  by  J.  Alexander.) 


12  COLLOID  CHEMISTRY 

With  the  reversible  colloids  (gelatin,  gum 
arabic,  albumen),  there  is  a  more  intimate 
union  between  the  two  phases;  in  fact  it  is 
probable  that  with  them  we  have  really  a 
mixture  of  (1)  a  dispersed  phase  of  water  sub- 
divided in  the  solid,  with  (2)  a  dispersing 
phase  of  the  solid  finely  subdivided  in  water. 
The  former  are  therefore  called  emulsoids  and 
the  latter  suspensoids.  Colloids  of  the  rever- 
sible type  are  also  said  to  be  hydrophile  or 
lyophile,  while  the  irreversible  colloids  are 
hydrophobe  or  lyophobe. 

No  sharp  line  is  to  be  drawn,  however,  for 
besides  intermediate  or  transition  cases  be- 
tween the  two  classes,  there  may  be  recognized 
two  groups  of  irreversible  colloids,  roughly 
defined  by  their  behavior  upon  concentration: 

First:  The  completely  irreversible ,  which 
coagulate  while  still  quite  dilute  and  separate 
sharply  from  the  solvent  with  the  formation  of 
a  pulverulent  precipitate  rather  than  a  gel 
(i.e.,  pure  colloidal  metals).  Chemical  or 
electrical  energy  is  needed  to  bring  them  back 
again  into  colloidal  solution. 

Second:  The  incompletely  reversible  which, 
when  quite  concentrated,  form  a  gel  that  may 


CLASSIFICATION  OF  COLLOIDS  13 

be  easily  redissolved  or  peptisized  by  com- 
paratively small  amounts  of  reagents,  unless 
the  evaporation  has  proceeded  too  far  (i.e., 
colloidal  stannic  acid). 


CHAPTER  III 
Consequences  of  Subdivision 

As  the  subdivision  of  a  substance  proceeds, 
the  area  of  its  effective  surface  increases  enor- 
mously, as  maybe  seen  from  the  following  Table 
III  adapted  from  Ostwald.  Consequently  sur- 
face forces,  such  as  adsorption,  capillarity  and 
surface  tension,  become  enormously  magnified 
and  of  primary  importance.  Furthermore,  the 
so-called  radius  of  molecular  attraction  (p  = 
50  MM)  is  well  within  the  colloidal  field,  so  that 
the  specific  attractive  forces  of  the  particles 
also  enter  as  a  controlling  factor.  In  fact, 
before  substances  can  unite  chemically  their 
particles  must  be  first  brought  into  proper 
subdivision  and  proximity,*  by  solution,  fusion, 
ionization  or  even  by  mere  pressure,  as  was 
demonstrated  by  W.  Spring,  who  caused  fine 

*  It  is  a  striking  fact  that  absolutely  dry  sodium  is  not 
attacked  by  absolutely  dry  chlorin.  M.  Raffo  and  A.  Pieroni 
observed  that  colloidal-  suphur  reduced  silver  salts  energeti- 
cally, whereas  even  fine  precipitated  sulphur  did  not  form 
silver  sulphid  in  the  cold,  and  did  so  only  partially  upon  boiling. 

14 


CONSEQUENCES  OF  SUBDIVISION 


15 


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16  COLLOID  CHEMISTRY 

dry  powders  to  combine  chemically  by  high 
pressure.  If  the  degree  of  subdivision  is  not 
profound  enough  to  permit  of  the  combination 
of  isolated  atoms  or  ions  with  each  other, 
chemical  combination  in  the  strict  sense  may 
not  occur,  but  there  may  be  produced  "  ad- 
sorption compounds  "  resulting  from  the  union 
of  atomic  or  ionic  mobs  in  indefinite  or  non- 
stoichiometric  proportions,  under  the  influence 
of  more  or  less  modified  chemical  forces.  The 
combination  of  arsenious  acid  and  ferric  oxid 
which  Bunsen  regarded  as  a  basic  ferric  arsen- 
ite,  4  Fe20s,  A^Oa,  5  H20,  has  been  shown  by 
Biltz  and  Behre  to  be  an  adsorption  compound; 
and  Zsigmondy  proved  "Purple  of  Cassius"  to  be 
an  adsorption  compound  of  colloidal  gold  and 
colloidal  stannic  acid  by  actually  synthesizing  it 
by  mixing  the  two  separate  colloidal  solutions. 
The  effect  of  increasing  subdivision  upon  the 
particles  in  colloidal  solutions  is  illustrated  in 
Table  IV,  adapted  from  Zsigmondy.  Tables 
V  and  VI  were  prepared  by  Zsigmondy  to 
illustrate  visually  the  relation  of  the  sizes  of 
colloidal  particles  to  well-known  microscopic 
objects  on  the  one  hand  and  to  the  theoretical 
sizes  of  molecules  on  the  other. 


TABLE  V 
LINEAR  MAGNIFICATION  1 :  10,000 


A.  Human  blood  corpuscles  (diameter  7.5  n,  thickness  1.6  p). 

B.  Fragment  of  rice  starch  granule  (according  to  v.  Hohnel)  3-8  /*. 

C.  Particles  in  a  kaolin  suspension. 

E.  Anthrax  bacillus  (length  4-15  n,  width  about  1  /*). 

F.  Cocci  (diameter  about  0.5-1  n,  rarely  2  /*)• 

f,  g,  h.  Particles  of  colloidal  gold  solutions  Au73a,  Au^,  Au«  ( 0.006-0.015  ft), 
i,  k,  1.   Particles  from  settled  gold  suspensions  (0.075-0.2  n). 


TABLE  VI 
LINEAR  MAGNIFICATION  1 :  1,000,000 


a       b 


?    D  m   S 

e      ei    e2      f  g 


n 


a-d.  —  Hypothetical  Molecular  Dimensions 

a.  Hydrogen  molecule  —  dia.  0.1  nil. 

b.  Alcohol  molecule  —  dia.  0.5  MM- 

c.  Chloroform  molecule  —  dia.  0.8  pp. 

d.  Molecule  of  soluble  starch  —  dia.  about  5  MM« 

e-h.—  Gold  Particles  in  Colloidal  Gold  Solutions 

e.  Gold  particle  in  Aui«  (too  small  to  determine), 
d.      "          "        "     "     ,  about  1.7  MM- 

ej.  "  "  "  "  ,  "  3.0  MM- 
/.  "  "  Au73a,  6  MM- 

g.  "  "  "  Au92,  "  10  MM- 
h.  "  "  "  AUOT,  "  15  MM- 
u  Gold  particle  in  settled  gold  suspension. 


CONSEQUENCES  OF  SUBDIVISION          17 


s 

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£ 


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CHAPTER  IV 
The  Ultramicroscope 

As  this  instrument  revolutionized  colloid 
research,  a  brief  description  of  it  is  essential. 

It  is  a  matter  of  every-day  experience  that  the 
unseen  motes  and  dust  particles  in  the  air  be- 
come visible  in  a  beam  of  bright  light,  espe- 
cially against  a  dark  ground,  and  in  this  simple 
fact  lies  the  principle  of  the  ultramicroscope. 

Faraday  and  later  Tyndall  made  use  of  a 
convergent  beam  of  light  to  demonstrate  the 
optical  inhomogeneity  of  solutions;  for  in 
fluids  not  optically  clear,  the  path  of  the  beam 
becomes  more  or  less  distinctly  visible,  because 
of  the  light  scattered  by  the  particles  present. 
In  this  manner  can  be  recognized  much  smaller 
quantities  of  matter  than  by  spectrum  analysis 
—  in  fact  less  than  10~8  mg.  (1/10,000,000)  of 
metallic  gold  can  thus  be  detected  with  the 
naked  eye. 

Prof.  Richard  Zsigmondy  while  experiment- 
ing with  colloidal  solutions  conceived  the  idea 
of  examining  this  light  cone  microscopically. 

18 


THE  ULTRAMICROSCOPE  19 

His  preliminary  experiments  having  demon- 
strated that  he  could  thus  see  the  individual 
particles  in  various  hydrosols,  he  sought  the 
assistance  of  Dr.  H.  Siedentopf,  scientific 
director  of  the  Carl  Zeiss  factory,  in  Jena, 
where  was  produced  the  first  efficient  ultra- 
microscope. 

The  ultramicroscope  consists  essentially  of 
a  compound  microscope  arranged  for  examin- 
ing in  a  dark  field  an  intense  convergent  beam 
of  light  cast  within  or  upon  the  substance 
under  examination.  The  light  seen  by  the 
eye  represents,  therefore,  the  light  diffracted, 
scattered  or  reflected  upward  by  the  substance 
or  by  particles  within  it. 

If  within  a  thin  beam  of  light  from  a  pro- 
jection lantern  we  scatter  successively  powders 
of  different  substances  in  various  degrees  of 
fineness  (mica  ground  to  pass  60,  100  and  160 
mesh;  lampblack;  powdered  oxid  of  zinc; 
flake  and  powdered  graphite),  some  of  them 
will  produce  only  a  homogeneous  illumination 
of  the  beam  in  which  no  isolated  particles  can 
be  seen,  whereas  with  others,  the  individual 
particles  are  distinctly  visible. 

Passing  the  beam  through  a  beaker  of  dis- 


20  COLLOID  CHEMISTRY 

tilled  water,  nothing  can  be  seen;  but  upon 
the  addition  of  a  faw  drops  of  colloidal  gold 
solution,  which  appears  quite  clear  to  trans- 
mitted light,  the  path  of  the  beam  through  the 
fluid  immediately  becomes  visible.  This  Tyn- 
dall  effect,*  as  it  is  called,  might  be  considered 
a  criterion  of  colloidal  solution  were  it  not  that 
very  minute  traces  of  colloidal  impurities  can 
produce  it  and  it  is  often  exhibited  by  solu- 
tions generally  regarded  as  crystalloidal  — 
those  of  many  dyestuffs  for  example;  further- 
more with  increasing  fineness  of  subdivision 
the  Tyndall  effect  decreases,  disappearing  as 
molecular  dimensions  are  approached. 

Just  as  in  the  cosmic  field  our  most  powerful 
telescopes  fail  to  resolve  the  fixed  stars,  which 
are  nevertheless  visible  as  points  of  light  of 
varying  brilliancy,  so,  too,  in  the  ultramicro- 
scopic  field,  we  can  see  particles  much  smaller 
than  the  resolving  power  of  the  microscope 
(that  is,  smaller  than  a  wave  length  of  light) 
provided  only  that  they  diffract  sufficient  light 
to  affect  the  retina.  Based  upon  the  experi- 
ence of  astronomers  we  may  be  able  greatly  to 
increase  the  sensitiveness  of  the  ultramicro- 

*  Also  known  as  the  Faraday-Tyndall  effect. 


THE  ULTRAMICROSCOPE  21 

scope  by  fortifying  the  eye,  so  to  speak,  with 
the  photographic  plate,  using  at  the  same  time 
tropical  sunlight  or  ultraviolet  light  for  illu- 
mination. 

In  the  original  form  of  the  ultramicroscope, 
as  perfected  by  Siedentopf  and  Zsigmondy, 
which  is  the  one  best  adapted  for  the  examina- 
tion of  transparent  solids,  a  side  illumination 
is  effected  by  a  microscope  objective  with 
micrometer  movements,  which  throws  an 
intense  but  minute  conical  beam  of  light  into 
the  fluid  contained  in  a  little  cell  having 
quartz  windows  at  the  side  and  top.  Above 
this  cell  a  compound  microscope  is  adjusted 
vertically,  so  that  the  narrowest  part  of  the 
light  cone  occupies  the  center  of  the  focal 
plane.  If  the  fluid  under  examination  is  op- 
tically clear  or  if  it  contains  particles  so  small 
that  they  cannot  diffract  sufficient  light  to 
create  a  visual  impression,  the  light  cone 
cannot  be  seen.  If  enough  light  is  diffracted, 
the  light  cone  becomes  visible,  being  homo- 
geneous if  the  particles  are  too  small  or  too 
close  together  to  be  individually  seen,  and 
heterogeneous  if  the  particles  can  be  individu- 
ally distinguished.  Particles  or  dimensions 


22  COLLOID  CHEMISTRY 

beyond  the  resolving  power  of  the  microscope 
(about  J  At)  are  for  brevity  termed  ultrami- 
crons.  Ultramicrons  that  can  individually  be 
made  visible  are  called  submicrons  (or  hypo- 
microns)  while  those  so  small  that  they 
produce  an  unresolvable  light  cone  are  termed 
amicrons. 

Knowing  the  percentage  of  gold  present  in 
a  colloidal  gold  solution  and  assuming  a  certain 
specific  gravity  and  uniform  shape  for  the  gold 
particles,  the  average  size  and  mass  of  a  single 
particle  of  colloidal  gold  can  be  calculated,  if 
the  number  present  in  a  given  volume  be  first 
counted.  In  this  manner  Zsigmondy  has 
shown  that  the  smallest  particles  of  colloidal 
gold  which  can  be  individually  distinguished 
with  bright  sunlight,  are  approximately  5  /*/* 
in  diameter,  that  is,  five-millionths  of  a  milli- 
meter; still  smaller  particles  exist  but  they 
produce  only  an  unresolvable  light  cone. 
Magnified  1,000,000  times  such  a  tiny  gold 
particle  would  be  about  J  inch  in  diameter, 
while  a  human  red  blood  corpuscle  would  be 
about  25  feet  across,  and  a  hydrogen  molecule 
a  speck  barely  visible.  The  gold  particles  in 
the  unresolvable  light  cone  must  therefore 


THE  ULTRAMICROSCOPE  23 

closely  approach  molecular  dimensions.  In 
fact,  by  allowing  amicrons  to  grow  into  visi- 
bility in  a  suitable  solution  and  then  counting 
them,  Zsigmondy  has  recently  shown  that 
some  of  the  particles  of  colloidal  gold  have 
a  mass  of  1-5. 10~16  mg.,  indicating  a  size  of  1.7 
to  3  jjifji. 

Various  other  types  of  ultramicroscopes, 
mainly  modifications  of  dark  field  illumination, 
have  been  developed  by  Cotton  and  Mouton, 
Ignatowski  (made  by  Leitz),  Siedentopf  (car- 
dioid  condenser,  made  by  Zeiss)  and  others, 
and  besides  being  useful  in  examining  colloidal 
solutions,  they  have  enabled  pathologists  to 
see  and  discover  ultramicroscopic  bacteria 
(spirochetes,  infantile  paralysis). 

Bausch  &  Lamb  Optical  Co.  of  Rochester, 
N.  Y.,  are  now  producing  a  useful  ultramicro- 
scope. 


CHAPTER  V! 
General  Properties  of  Colloids 

The  optical  properties  of  colloids  and  their 
simulation  of  chemical  compounds  have  been 
already  referred  to.  The  other  general  proper- 
ties of  colloids  may  be  considered  under  the 
following  headings: 

1.  Colloidal  Protection. 

2.  Dialysis,  Ultrafiltration  and  Diffusion. 

3.  Electric  Charge  and  Migration. 

4.  Pectization  (Coagulation)  and  Peptiza- 

tion. 

COLLOIDAL  PKOTECTION.  —  A  most  inter- 
esting and  important  fact  regarding  reversible 
colloids  is  that  they  can  communicate  their  re- 
versible property  to  irreversible  colloids.  The 
addition  of  gelatin  (as  little  as  0.0001  per  cent) 
to  a  solution  of  colloidal  gold  protects  the  latter 
against  coagulation  upon  the  addition  of 
electrolytes,  and  permits  it  to  redissolve  after 
desiccation.  Ultramicroscopic  examination 
has  shown  that  the  gelatin  does  not  affect  the 

24 


GENERAL  PROPERTIES  OF  COLLOIDS        25 

motility  of  the  gold  particles,  thus  disposing  of 
the  suggestion  of  Lobry  de  Bruyn  that  it  acts 
by  decreasing  their  motion.  The  idea  ad- 
vanced by  Miiller  (Ber.,  1904,  37,  11)  that 
gelatin  acts  by  increasing  the  viscosity  and 
thus  preventing  the  deposition  of  suspended 
particles  is  disproved  by  the  fact  that  quince 
kernel  gum,  notwithstanding  its  viscosity, 
exercises  no  protective  action,*  whereas  the 
small  quantities  of  gelatin  necessary  to  pro- 
duce this  effect  cannot  appreciably  increase 
the  viscosity,  and  actually  permit  gold  par- 
ticles to  settle  if  they  are  large  enough  to  do  so. 

The  action  of  reversible  colloids  in  opposing 
group  formation,  is  of  great  technical  impor- 
tance, for  in  many  cases  it  permits  them  to 
hinder,  modify  and  even  prevent  coagulation, 
precipitation  and  crystallization. 

DIALYSIS.  —  Colloid  solutions  possess  a  small 
but  definite  diffusibility  through  colloidal  septa 
(parchment  paper,  bladder)  as  was  recognized 
by  Graham,  who  found  that  "tannic  acid 
passes  through  parchment-paper  about  200 
times  slower  than  sodium  chlorid;  gum  arabic 

*  According  to  Zsigmondy,  quince  kernel  gum  acts  as  a 
protector  with  some  substances. 


26  COLLOID  CHEMISTRY 

400  times  slower/'  Graham's  original  form  of 
dialyzer  may  be  made  from  a  wide-mouthed 
bottle  whose  bottom  has  been  removed.*  The 
mouth  is  closed  by  a  piece  of  bladder  or  parch- 
ment paper  tightly  bound  on,  the  solution  to 
be  dialyzed  is  poured  in,  and  the  bottle  im- 
mersed about  halfway  in  water  contained  in  a 
larger  vessel.  Most  of  the  crystalloids  diffuse 
through  the  membrane  into  the  outer  water, 
which  should  be  frequently  renewed,  while 
most  of  the  colloids  remain  in  the  original 
bottle,  and  may  be  thus  obtained  in  a  purified 
condition.  Improved  modern  dialyzers  con- 
sist of  parchment  or  collodion  sacs  or  thimbles, 
or  even  of  whole  bladders,  which  have  the 
advantage  of  a  larger  dialyzing  surface. 

ULTRAFILTRATION. — H.  Bechhold  found  that 
he  could  make  filtering  membranes  of  varying 
degrees  of  permeability  by  forming  them  from 
jellies  of  varying  concentration.  He  used  prin- 
cipally collodion  dissolved  in  glacial  acetic  acid 
and  afterward  immersed  in  water,  and  gelatin 
jellies  hardened  in  ice-cold  formaldehyde.  The 
jellies  were  formed  and  hardened  on  pieces  of 
filter  paper,  which  were  supported  from  below 

*  A  lamp  chimney  will  answer  very  well. 


GENERAL  PROPERTIES  OF  COLLOIDS       27 

by  nickel  wire  cloth,  and  clamped  between 
two  flanges.  The  liquid  to  be  subjected  to 
ultrafiltration  is  introduced  in  the  chamber 
thus  formed  and  forced  through  the  prepared 
septum  by  appropriate  pressure,  which  may 
run  up  to  20  atmospheres  or  more  and  may 
be  produced  by  a  pump  or  by  compressed  gas 
(air,  nitrogen  or  CO2).  Table  VII  (p.  28),  pre- 
pared by  Bechhold,  shows  various  colloids  ar- 
ranged in  order  of  the  diminishing  size  of 
their  particles  in  solution,  and  was  obtained 
by  using  ultrafilters  of  varying  degrees  of 
porosity  or  permeability. 

By  means  of  ultrafiltration  through  ultra- 
filters  of  appropriate  permeability,  not  only 
may  colloids  be  separated  from  crystalloids, 
but  colloids  having  particles  of  different  sizes 
may  be  separated  from  each  other. 

DIFFUSION.  —  Diffusion  through  a  septum 
is,  of  course,  involved  in  dialysis.  If,  however, 
diffusion  occurs  into  a  jelly,  many  interest- 
ing phenomena  may  develop,  especially  if  the 
jelly  adsorbs  arty  of  the  diffusing  substances 
or  contains  substances  which  can  react  with 
them. 

Owing  to  the  enormous  surface  they  present, 


28  COLLOID  CHEMISTRY 

TABLE  VII 

Suspensions. 

Prussian  blue. 

Platinum  sol  (made  by  Bredig's  method). 

Ferric  oxid  hydrosol. 

Casein  (in  milk). 

Arsenic  sulphid  hydrosol. 

Colloidal    gold   hydrosol    (Zsigmondy's    No.    4,    particles 

about  40  MM). 

Colloidal  bismuth  oxid  (Paal's  "Bismon"). 
Colloidal  silver  (Paal's  "Lysargin"). 
Colloidal  silver  (von  Heyden's  "Collargol,"  particles  about 

20  MM). 
Colloidal   gold   hydrosol    (Zsigmondy's    No.    0,    particles 

about  1-4  MM). 
Gelatin  solution,  1  per  cent. 
Hemoglobin  solution,  1  per  cent  (molecular  weight  about 

16,000). 

Serum  albumin  (molecular  weight  about  5000  to  15,000). 
Diphtheria  toxin. 
Protalbumoses. 
Colloidal  silicic  acid. 
Lysalbinic  acid. 
Deuteroalbumoses  A. 

Deuteroalbumoses  B  (molecular  weight  about  2400). 
Deuteroalbumoses  C. 
Litmus. 

Dextrin  (molecular  weight  about  965). 
Crystalloids. 

colloidal  gels  exhibit  a  powerful  adsorptive 
action.  In  fact,  even  when  percolated  through 
such  a  relatively  coarse-grained  septum  as  sand, 
most  solutions  issue  with  a  materially  reduced 


GENERAL  PROPERTIES  OF  COLLOIDS       29 

content  of  solute,  and  benzopurpurin  solutions 
may  be  thus  decolorized.  Further,  if  a  solute 
hydrolyzes  into  ions  having  different  degrees 
of  adsorbability  or  different  rates  of  diffusibil- 
ity,  they  may  be  actually  separated  by  diffusion 
through  a  colloidal  gel. 

This  phenomenon  is  nicely  exhibited  by 
what  may  be  termed  a  " patriotic  test  tube," 
prepared  by  filling  a  tube  about  two-thirds  full 
with  a  slightly  alkaline  solution  of  agar  contain- 
ing a  little  potassium  ferrocyanid  and  enough 
phenolphthalsin  to  turn  it  pink.  After  the  agar 
has  set  to  a  firm  gel,  a  solution  of  ferric  chlorid 
is  carefully  poured  on  top,  and  almost  instantly 
the  separation  becomes  evident.  The  iron 
forms  with  the  ferrocyanid  a  slowly  advancing 
band  of  blue,  before  which  the  more  rapidly 
diffusing  hydrochloric  acid  spreads  a  white 
band  as  it  discharges  the  pink  of  the  indicator. 
After  the  lapse  of  a  few  days  the  tube  is  about 
equally  banded  in  red,  white,  and  blue. 

Even  then  the  tubes  do  not  cease  to  be  of 
interest,  for  if  they  are  allowed  to  stand  several 
weeks  the  pink  color  is  all  discharged  and  there 
develop  peculiar  bands  or  striations  of  blue, 
apparently  due  to  the  fact  that  the  iron  ferro- 


30  COLLOID  CHEMISTRY 

cyanid  temporarily  blocks  the  diffusion  pas- 
sage, which  are  gradually  opened  again  after 
a  layer  of  the  blue  salt  has  diffused  on  from  the 
lower  surface. 

f  Not  only  may  ions  be  thus  separated,  but  if 
two  solutes  in  the  same  solvent  possess  differ- 
ent rates  of  diffusion  or  different  degrees  of 
adsorbability,  they  also  may  be  separated  from 
each  other  by  diffusion  through  a  colloidal  gel 
or  septum.  (Differential  Diffusion.) 

ELECTRIC  CHARGE  AND  MIGRATION.  —  The 
particles  of  practically  all  colloidal  solutions 
possess  an  electric  charge,  and  under  the 
influence  of  an  electric  current  (difference  of 
potential)  move  toward  the  electrode  having 
the  opposite  charge.  (Electrophoresis.)  In 
general,  when  two  substances  are  brought  into 
contact,  the  one  having  the  higher  dielectric 
constant  becomes  positively  charged,  whereas 
the  one  with  the  lower  dielectric  constant 
becomes  negatively  charged  (Cohen's  Law). 
Since  water  has  a  high  dielectric  constant  (80), 
most  substances  suspended  in  pure  water 
become  negatively  charged  and  wander  to  the 
anode.  On  the  other  hand  if  suspended  in  oil 
of  turpentine,  which  has  a  low  dielectric 


GENERAL  PROPERTIES  OF  COLLOIDS       31 

constant  (2.23),  they  become  positively  charged 
and  wander  to  the  cathode. 

If,  however,  electrolytes  are  present,  Coehn's 
law  is  superseded  by  other  controlling  factors, 
such  as  the  adsorption  of  ions,  which  may  give 
their  charge  to  the  suspended  particles.  In 
fact  Hardy  found  that  in  pure  water  albumen 
was  amphoteric;  in  the  presence  of  a  trace  of 
alkali  it  acquired  a  negative  charge  and 
migrated  to  the  anode;  but  a  trace  of  acid  gave 
it  a  positive  charge  and  it  then  migrated  to  the 
cathode.  The  following  table  shows  the  usual 
charge  and  migration  tendency  of  a  number 
of  aqueous  colloidal  solutions. 

Charged  +  Charged  — 

Migrate  to  Cathode  (-  Pole)  Migrate  to  Anode  (4-  Pole) 


1.  Hydrates  of  Fe,  Cu,  Cd,  Al,  Zr,  1.  Sulphids  of  As,  Sb,  Cu,  Pb,  Gd. 

Ce,  Th.  Halides  of  Ag. 

2.  Titanic  acid.  2.  Stannic  acid,  silicic  acid. 

3.  Colloidal    Bi,  Pb,  Fe    and    Gu  3.  Colloidal  Pt,  Au,  Ag,  and  Hg, 

(Bredig's  method).  I,  S,  Se. 

4.  Albumen,  hemoglobin,  agar.  4.  Gum     arabic,     soluble     starch, 

gamboge,  mastic,  oil  emulsion. 

5.  Basic     Dyes:      Methyl     violet,  5.  Acid     Dyes:      Eosin,     fuchsin, 

Bismarck    brown,     methylen  anilin     blue,    indigo,    soluble 

blue,  Hofmann  violet.  Prussian  blue. 

PECTIZATION  AND  PEPTIZATION.  —  Briefly 
stated  pectization  means  the  coagulation  of  a 
colloidal  sol,  and  peptization  its  redispersion. 
If  a  small  quantity  of  an  electrolyte  is  added 


32  COLLOID  CHEMISTRY 

to  a  pure  ruby  red  colloidal  gold  solution,  the 
latter  changes  to  a  blue  or  violet  color,  and 
deposits  its  gold  as  a  fine  blackish  coagulum 
or  precipitate.*  By  watching  in  the  ultra- 
microscope  the  coagulation  of  very  dilute 
milk  by  dilute  acid,  the  individual  particles  of 
the  colloidal  casein  may  be  seen  to  gather 
gradually  together  into  groups,  whose  motion 
becomes  progressively  less  as  their  size  in- 
creases, until  they  are  no  longer  able  to  stay 
afloat,  and  finally  coagulate  in  large  grape-like 
clusters.  Hardy  believes  that  the  particles  of 
colloids  adsorb  the  oppositely  charged  ions  of 
electrolytes  present;  at  the  isoelectric  point 
(that  is  when  there  is  no  excess  either  of  posi- 
tive or  negative  charges  on  the  particles)  coag- 
ulation occurs.  If,  however,  an  excess  of  elec- 
trolyte be  added  all  at  once,  the  isoelectric 
point  may  be  passed  before  coagulation  occurs, 
and  the  particles  acquire  a  charge  opposite  to 
the  one  they  had  originally.  Under  such  con- 
ditions, no  coagulation  may  result. 

*  The  amount  in  milligrams  of  protective  colloid  just 
sufficient  to  prevent  the  change  to  violet  of  10  cc.  of  bright 
red  colloidal  gold  solution  by  the  addition  of  1  cc.  of  a  10 
per  cent  solution  of  NaCl,  is  called  the  "gold  figure"  or  "gold 
number"  of  the  protector. 


GENERAL  PROPERTIES  OF  COLLOIDS        33 

Burton  epitomizes  the  difference  in  action 
of  various  electrolytes  as  follows:  "Two 
remarkable  results  are  evident  on  comparing 
the  coagulative  powers  of  various  electrolytes 
on  colloids  of  different  kinds;  first,  the  coagu- 
lation depends  entirely  on  the  ion  bearing  a 
charge  of  sign  opposite  to  that  of  the  colloidal 
particle;  and,  second,  with  solutions  of  salts, 
trivalent  ions  have,  in  general,  immensely 
greater  coagulative  power  than  divalent  ions, 
and  the  latter,  in  turn,  much  greater  than 
univalent.  Acids  and  alkalis  in  particular 
cases  act  more  strongly  than  the  corresponding 
salts." 

High-tension  electric  discharges  may  also 
effect  the  coagulation  or  precipitation  of  a 
finely  subdivided  or  dispersed  phase;  which 
fact  was  utilized  by  Sir  Oliver  Lodge  in  dis- 
pelling fogs,  and  by  Cottrell  hi  coagulating 
smelter  and  similar  fumes. 

PEPTIZATION.  —  So  strong  is  the  analogy 
between  digestion  and  colloidal  disintegration 
that  Thomas  Graham,  the  father  of  colloid 
chemistry,  coined  the  word  peptization  to 
express  the  liquefaction  of  a  gel.  He  first 
speaks  of  the  coagulation  or  pectization  of 


34  COLLOID  CHEMISTRY 

colloids.  "The  pectization  of  liquid  silicic 
acid,"  he  states,  "and  many  other  liquid 
colloids  is  effected  by  contact  with  minute 
quantities  of  salts  in  a  way  which  is  not  under- 
stood. On  the  other  hand,  the  gelatinous  acid 
may  be  again  liquefied,  and  have  its  energy 
restored  by  contact  with  very  moderate 
amounts  of  alkali.  The  latter  change  is 
gradual,  1  part  of  caustic  soda,  dissolved  in 
10,000  water,  liquefying  200  parts  of  silicic 
acid  (estimated  dry)  in  60  minutes  at  100 
degrees.  Gelatinous  stannic  acid  also  is  easily 
liquefied  by  a  small  proportion  of  alkali,  even 
at  the  ordinary  temperature.  The  alkali,  too, 
after  liquefying  the  gelatinous  colloid,  may  be 
separated  again  from  it  by  diffusion  into  water 
upon  a  dialyzer.  The  solution  of  these  col- 
loids in  such  circumstances  may  be  looked 
upon  as  analogous  to  the  solution  of  insoluble 
organic  colloids  witnessed  in  animal  digestion, 
with  the  difference  that  the  solvent  fluid  here 
is  not  acid  but  alkaline.  Liquid  silicic  acid 
may  be  represented  as  the  'peptone'  of 
gelatinous  silicic  acid;  and  the  liquefaction  of 
the  latter  by  a  trace  of  alkali  may  be  spoken  of 
as  the  peptization  of  the  jelly.  The  pure 


GENERAL  PROPERTIES  OF  COLLOIDS       35 

jellies  of  alumina,  peroxide  of  iron  and  titanic 
acid,  prepared  by  dialysis,  are  assimilated 
more  closely  to  albumen,  being  peptized  by 
minute  quantities  of  hydrochloric  acid." 

Peptization  is  in  reality  deflocculation,  a 
dispersion  of  groups  into  separate  particles 
which  once  more  acquire  active  motion  and 
remain  afloat  or  in  solution.  The  detergent 
action  of  soap  and  dilute  alkalis  is  due  to  the 
fact  that  they  deflocculate  adhering  particles 
of  "  dirt." 


CHAPTER  VI 

Practical    Applications    of    Colloid    Chemical 
Principles 

The  practical  applications  of  colloid  chemis- 
try are  so  manifold  and  widespread  that  they 
touch  every  branch  of  science  and  technology. 
Whole  books  may  be  and  have  been  written  on 
many  of  the  most  restricted  fields,  while  the 
scientific  literature  teems  with  monographs 
and  articles,  directly  on,  or  applicable  to, 
colloid-chemical  subjects.  In  what  follows,  it 
will  be  possible  therefore  to  give  not  an  ex- 
haustive, but  only  a  most  general  survey, 
intended  rather  to  show  the  ubiquity  of  col- 
loid phenomena;  and  many  important  topics 
must  be  dismissed  with  a  most  rudimentary 
discussion,  altogether  incommensurate  with 
theu*  importance. 

ASTRONOMY.  —  As  matter  in  colloidal  state 
is  so  common  on  our  relatively  minute  earth,  it 
is  but  natural  to  expect  to  find  many  instances 
of  colloidal  dispersion  in  the  immensity  of  the 
Universe. 

36 


PRACTICAL  APPLICATIONS  37 

Cosmic  dust  Is  widely  distributed  throughout 
space,  and  as  it  is  gathered  up  by  the  superior 
attraction  of  the  larger  heavenly  masses  (suns, 
planets,  etc.),  which  in  any  system  grow  at  the 
expense  of  the  smaller  masses,  fresh  quantities 
are  continually  produced  by  the  collisions  of 
bodies  in  space,  as  well  as  the  disintegration 
of  meteorites,  comets,  asteroids,  etc. 

The  tails  of  comets  seem  to  consist  almost 
entirely,  and  the  nuclei  and  coma  largely,  of 
colloidally  dispersed  matter.  The  great  comet 
of  1882  which  made  a  transit  of  the  sun,  was 
invisible  against  the  solar  disc  (a  position  corre- 
sponding to  attempted  observation  of  colloidal 
particles  in  the  ordinary  microscope  against  a 
luminous  background),  but  became  visible 
again  after  passing  beyond  the  sun's  disc  (a 
position  corresponding  to  successful  observa- 
tion of  the  same  colloidal  particles  in  the  ultra- 
microscope  against  a  dark  background,  the  eye 
of  the  observer  being  protected  from  the  source 
of  illumination). 

The  streaming  of  the  cometary  tails  away 
from  the  sun  may  be  due  to  the  ionization  of 
the  constituent  colloidal  particles,  and  their 
consequent  electrical  repulsion;  or  more  prob- 


38  COLLOID  CHEMISTRY 

ably,  it  may  be  due  to  the  sun's  rays,  as  was 
pointed  out  by  J.  Clerk  Maxwell.  The  inten- 
sity of  the  action  of  the  sun's  rays  on  a  particle 
depends  upon  its  surface,  which  varies  as  the 
square  of  its  diameter,  whereas  the  gravitation 
of  the  same  particle  to  the  sun  depends  upon  its 
mass,  which  varies  as  the  cube  of  its  diameter. 
Theoretically  in  the  case  of  a  particle  whose 
density  equals  that  of  water,  the  repulsion 
balances  gravitation  when  the  diameter  reaches 
0.0015  mm.  (=  1.5  /*).  As  the  diameter  di- 
minishes the  repulsive  force  gains  the  ascend- 
ancy, soon  reaching  a  maximum  and  again 
diminishing,  until  when  the  particle  has  a 
diameter  of  only  0.00007  (=  70  MM)  the  two 
forces  again  balance  each  other.* 

These  figures,  which  refer  to  a  substance  hav- 
ing the  density  of  water,  are  approximately  of 
colloidal  dimensions;  but  in  the  case  of  denser 
bodies  the  subdivision  would  be  even  more 
profound.  It  is  therefore  not  surprising  that, 
when  the  earth  recently  passed  through  the  tail 
of  a  comet,  no  disturbance  of  any  kind  was 

*  See  Simon  Newcomb's  article  on  "Comet,"  Encyclo- 
pedia Britannica,  llth  edition.  Also  Svante  Arrhenius, 
"  Worlds  in  the  Making,"  Harper  &  Bros. 


PRACTICAL  APPLICATIONS  39 

noticed.  The  comet's  tail  is  a  vast  celestial 
camouflage  —  its  luminosity  a  macroscopic  Far- 
aday-Tyndall  effect. 

The  nebulae,  too,  apparently  consist  of  finely 
dispersed  matter,  rendered  luminous  by  neigh- 
boring suns;  although  with  them  as  with  the 
comets,  a  small  part  of  the  light  may  result 
from  self -luminescence  (incandescent  gas,  etc.). 

METEOROLOGY.  —  What  we  commonly  call 
"  weather  conditions  "  are  largely  dependent 
upon  the  degree  of  dispersion  of  water  in  the 
atmosphere,  and  this  dispersion  is  mainly 
effected  and  maintained  by  solar  heat  and 
electrical  energy.  When  air  carrying  water 
vapor  is  chilled  by  rising  to  a  higher  level, 
meeting  a  colder  mass  of  air,  or  even  by  the 
alternation  of  night  and  day,  the  moisture 
it  contains  assumes  the  colloidal  state  as  cloud, 
fog  or  mist;  and  as  the  coagulation  of  the 
dispersed  water  proceeds,  these  in  turn  may 
condense  still  further  into  dew,  rain,  snow  or 
hail,  depending  upon  conditions.  When  the 
dispersed  water  aggregates,  there  is  naturally 
set  free  the  energy  originally  used  in  its  disper- 
sion, and  this  may  appear  as  electricity  (light- 
ning) especially  if  the  aggregation  occurs 


4:0  COLLOID  CHEMISTRY 

suddenly  as  is  the  case  in  thunder  and  hail 
storms.  We  have  all  noticed  how  a  nearby 
lightning  flash  is  promptly  followed  by  an 
increased  fall  of  raindrops. 

Were  it  not  for  our  atmosphere,  the  sun  would 
appear  to  us  like  a  fiery  ball  set  in  a  black  star- 
sprinkled  sky.  The  blue  color  of  the  sky  is  due 
to  diffraction  of  the  sunlight  by  the  earth's 
atmosphere,  a  gigantic  Tyndall  effect.  If  we 
look  edgewise  through  a  clear  sheet  of  glass,  we 
at  once  notice  the  green  color  due  to  colloidally 
dispersed  iron,  and  in  like  manner,  if  we  look 
through  a  great  length  of  the  atmosphere  the 
prevailing  color  is  blue.  As  the  poet  Campbell 
beautifully  puts  it : 

!<  Tis  distance  lends  enchantment  to  the  view, 
And  robes  the  mountain  in  its  azure  hue." 

After  the  tremendous  explosive  eruption  of 
the  volcano  Krakatoa  in  1883,  colloidal  dust 
and  ashes  were  projected  so  high  that  they 
gradually  spread  around  the  earth,  causing 
" golden  sunsets." 

We  do  not  know  to  what  extent  electrical 
conditions  on  the  earth  affect  the  dispersion  of 
substances  in  its  atmosphere;  but  since  half 
of  the  earth  is  always  heated  by  the  sun  while 


PRACTICAL  APPLICATIONS  41 

the  other  half  is  cooler,  thermoelectric  currents 
are  continually  circulating  about  the  earth. 
Variations  in  solar  radiation  due  to  sun-spots 
and  the  like,  cause  violent  electric  and  magnetic 
storms  which  are  intimately  connected  with  the 
aurora,  and  other  atmospheric  phenomena 
(ionization,  electrical  charge  of  dispersed  par- 
ticles); and  it  is  well  known  that  sun-spots 
exercise  a  potent  influence  on  the  weather. 

GEOLOGY  AND  MINERALOGY.  -  -  The  ordi- 
nary properties  of  the  solid  constituents  of  the 
earth's  crust  depend  more  upon  their  state  of 
physical  subdivision  than  upon  their  chemical 
constitution.  Atterberg  classified  the  frag- 
ments of  minerals  and  rocks  as  follows: 

Diameter. 

Boulders 2  m.  to  20  cm. 

Pebbles 20  cm.  to  2  cm. 

Gravel 2  cm.  to  2  mm. 

Sand 2  mm.  to  0.2  mm. 

Earth 0.2  mm.  to  0.02  mm. 

Loam 0.2  mm.  to  0.002  mm. 

Clay smaller  than  0.002  mm. 

The  smaller  the  particles,  the  greater  their 
capillarity  and  the  ease  with  which  they  are 
moved  by  wind  and  by  water,  but  the  less 
their  permeability  to  water.  Fine  defloccu- 


42  COLLOID  CHEMISTRY 

lated  clay  is  carried  thousands  of  miles  by 
rivers  until  it  is  finally  coagulated  by  the  salts 
of  the  ocean,  as  may  be  observed  in  the  deltas 
of  the  Ganges,  Nile  and  Mississippi.  Fine 
particles  are  easily  cemented  by  pressure  or 
igneous  action  into  rocks  (e.g.,  sandstone, 
slate),  or  may  act  as  a  cement  for  large  par- 
ticles (e.g.,  pudding-stone)  or  as  a  matrix  for 
fossils. 

Many  minerals  are  themselves  colloidal  gels 
(e.g.,  opal,  flint,  bauxite)  or  result  from  the 
weathering  of  other  minerals  with  consequent 
gel  formation  (e.g.,  kaolin  from  kaolinite, 
serpentine  from  diabase).  Most  gems  owe 
their  colors  to  impurities  colloidally  dispersed 
within  them  (e.g.,  ruby,  emerald,  amethyst). 
Dendrites  are  formed  by  solutions  diffusing 
through  mineral  gels.  Colloidal  minerals  usu- 
ally adsorb,  and  are  dyed  by  aniline  dyes 
(methylen  blue),  whereas  crystalloid  minerals 
are  unaffected. 

CLAY  AND  CEEAMICS.  —  The  effect  of  vege- 
table extractive  matters  on  the  working 
properties  of  clay  have  been  known  from 
ancient  times  —  in  the  Bible  (Exodus  V)  it 
is  mentioned  that  brick  cannot  be  made  with- 


PRACTICAL  APPLICATIONS  43 

out  straw.  Recently  patents  have  been  taken 
out  for  "  Egyptianizing  "  clay  by  adding  to  it 
tannin,  extract  of  straw,  humus  and  the  like. 
Glue  and  similar  protective  colloids  defloccu- 
late  or  "free  out  "  clay  and  make  it  " cover  " 
in  paper-coating  and  kalsomining.  The  work- 
ing properties  of  clays  depend  largely  upon  the 
size  of  their  constituent  particles  and  their 
state  of  aggregation.  This  is  especially  evi- 
dent in  ceramics.  Articles  molded  of  clay  and 
then  burned,  lose  their  hydrosol  condition  and 
become  hardened  into  pottery. 

AGRICULTURE.  —  Although  from  time  im- 
memorial farmers  have  classified  soils  on  the 
basis  of  their  physical  and  physiological 
character  as  "light "  or  "heavy,"  "rich  "  or 
"poor,"  "productive"  or  "unproductive," 
etc.,  it  is  only  within  comparatively  recent 
years  that  chemists  have  begun  to  realize  the 
full  importance  of  the  role  played  by  the 
colloids,  especially  the  organic  colloids  of  the 
soil. 

Many  important  properties  of  soils,  such  as 
permeability,  capillarity,  absorption,  moisture 
content,  etc.,  are  dependent  not  so  much  upon 
the  chemical  composition  as  upon  the  size  of 


44  COLLOID  CHEMISTRY 

the  constituent  soil  particles.  (See  Atterberg, 
Schwed,  landw.  Akad.,  1903,  and  Chem.  Zeit.. 
1905,  29,  195;  Patten  and  Waggaman,  U.  S. 
Dept.  of  Agri.  Bureau  of  Soils,  Bull.  No.  52, 
1908).  In  coarse  sand,  for  example,  the 
amount  of  water  is  greatest  at  the  bottom  and 
smallest  at  the  top,  whereas  in  fine  clay  the 
distribution  is  much  more  uniform. 

Among  the  natural  agencies  tending  to 
increase  the  size  of  the  minute  soil  particles 
may  be  mentioned  heat  with  its  drying  or 
evaporative  effect,  freezing,  and  the  coagulat- 
ing or  flocculating  action  of  soluble  inorganic 
salts  and  some  organic  substances  present  in 
the  soil.  On  the  other  hand,  included  in  that 
little  known  class  of  substances  vaguely  de- 
scribed as  " humus,"  there  are  numerous 
organic  substances  derived  from  the  bacterial, 
plant,  or  animal  debris,  or  exuded  by  the  roots 
of  plants,  which  act  as  protective  colloids 
(Schutzkolloide)  and  tend  to  produce  and 
maintain  the  hydrosol,  or  deflocculatd  con- 
dition. (See  P.  Ehrenberg,  "Die  Kollide  des 
Ackerbodens,"  Zeits.  angew.  Chem.,  1908, 
41,  2122.)  In  an  excellent  paper  on  the 
mechanics  of  soil  moisture,  L.  J.  Briggs  (U.  S. 


PRACTICAL  APPLICATIONS  45 

Dept.  of  Agric.,  Bureau  of  Soils,  Bull.  No. 
10, 1897)  pointed  out  that  very  small  quantities 
of  certain  organic  substances,  such  as  are  con- 
tinually being  produced  in  the  soil  by  the 
decay  of  organic  matter,  greatly  decrease  the 
surface  tension  of  solutions,  thus  counteracting 
to  a  large  extent  the  effects  of  the  surface 
application  of  soluble  salts  which  would  tend 
to  draw  moisture  to  the  surface  by  increasing 
the  surface  tension  of  the  capillary  water  of 
soils.  It  is  well  known,  however,  that  an 
excess  of  salts  will  ruin  a  soil  physically,  as  is 
evident  after  flooding  by  sea  water  or  the 
excessive  application  of  chemical  fertilizers. 
Of  interest  in  this  connection  is  the  recent  work 
of  the  Bureau  of  Soils,  U.  S.  Department 
of  Agriculture,  carried  out  by  Cameron, 
Schreiner,  Livingston  and  their  co-workers. 
Thus  plants  grown  in  the  unproductive  Ta- 
koma  soil,  were  greatly  benefitted  by  green 
manure,  oak  leaves,  tannin  and  pyrogallol. 
The  injurious  effects  of  quinone  and  some 
other  organic  substances  may  be  due  to  their 
ability  to  precipitate  or  flocculate  the  pro- 
tective colloids  of  the  soil;  for  as  Lumiere 
and  Seyewetz  have  shown  (Bull.  Soc.  Chim., 


46  COLLOID  CHEMISTRY 

1907,  4,  428-431;  J.  S.  C.  L,  1907,  703) 
quinone  renders  gelatin  insoluble. 

The  fact  observed  by  Fickenday  (J.  Landw., 
1906,  64,  343)  that  more  alkali  is  required  to 
flocculate  natural  clay  soils  than  kaolin  sus- 
pensions, he  attributes  to  the  protective  action 
of  the  humus  present  (see  Keppeler  and  Spang- 
enberg,  J.  Landw.,  1907,  55,  299). 

A.  S.  Cushman,  in  his  excellent  work  upon 
the  use  of  feldspathic  rock  as  fertilizer  (U.  S. 
Dept.  of  Agriculture,  Bureau  of  Plant  Indus- 
try, Bulletin  No.  104;  Cushman  and  Hubbard, 
J.  Am.  Chem.  Soc.,  30,  779),  has  shown  that 
the  fine  grinding  of  feldspar  increases  the 
amount  of  potash  available  under  the  action  of 
water.  Thus,  a  coarse  powder  having  an 
area  of  43  sq.  cm.  per  cc.  of  solid  feldspar 
yielded  0.013  per  cent,  whereas  a  fine  powder 
whose  area  was  501,486  sq.  cm.  per  cc.  yielded 
0.873  per  cent  of  potash  and  soda.  These  fine 
particles  averaged  about  0.1  /*  in  diameter, 
which  is  relatively  large  as  compared  with 
colloidal  dimensions;  but  under  the  action  of 
physical  and  chemical  soil  agencies  they 
undergo  further  disintegration,  finally  reaching 
a  colloidal  condition  in  which  still  more  of 


PRACTICAL  APPLICATIONS  47 

their  potash  is  available,  a  condition  favored 
and  maintained  by  the  organic  protective 
colloids  of  the  soil. 

With  these  brief  and  inadequate  remarks  we 
must  dismiss  this  subject  of  such  vast  impor- 
tance and  fascinating  interest,  referring  to  the 
extensive  literature,  much  of  which  is  quoted 
in  Bulletin  No.  52  and  the  other  publications 
of  the  Bureau  of  Soils. 

ELECTROPLATING  AND  ELECTRODEPOSITION  OF 
METALS.  —  The  addition  of  protective  colloids 
to  electroplating  baths  tends  to  the  production 
of  fine-grained  non-crystalline  deposits.  A.  G. 
Betts  in  a  paper  entitled  "The  Phenomena  of 
Metal  Depositing "  (J.  Am.  Electrochem.  Soc., 

1905,  8,  63)  has  shown  that  there  are  many 
factors  influencing  the  action  of  the  colloid, 
and  has  suggested  a  number  of  possible  ex- 
planations.    The    correct    explanation,    how- 
ever, has  been  given  by  Mtiller  and  Bahntje 
(Z.  Elektrochem.,  1906,  12,  317;  J.  S.  C.  I., 

1906,  484)  who  state  that  the  added  colloid 
keeps   the    deposited   metal    (copper)    in   an 
amorphous,  non-crystalline  condition,  gelatin 
producing  the  most  powerful  effect,  egg  al- 
bumen   considerably    less,    while    gum    and 


48  COLLOID  CHEMISTRY 

starch  have  comparatively  little  action.  They 
also  found  that  the  deposited  copper  weighed 
about  0.2  per  cent  more  than  under  normal 
conditions,  indicating  that  some  of  the  colloid 
had  been  carried  down  with  the  metal. 

The  relative  efficiency  of  the  colloids  just 
referred  to  corresponds  to  their  relative  effi- 
ciency in  protecting  from  coagulation  solutions 
of  colloidal  gold  (see  Zsigmondy,  J.  S.  C.  I., 
1902,  192;  also  Colloids  and  the  Ultramicro- 
scope,  p.  81),  which  is  additional  evidence 
that  we  have  another  instance  of  protective 
colloidal  action,  by  which  the  crystallization 
forces  of  the  metal  are  powerfully  influenced. 

METALLURGY.  —  Since  coarsely  crystalline 
metals  are  brittle,  tending  to  split  along  the 
lines  of  crystal  cleavage,  various  physical  and 
chemical  means  are  employed  in  technical 
practice  to  obtain  a  hard,  fine-grained  struct- 
ure. (See  I.  Langmuir,  Iron  &  Steel  Inst., 
Sept.  1907;  J.  S.  C.  I.,  1907,  1094.)  Among 
the  physical  methods  are  chilling  and  rolling, 
while  the  chemical  methods  involve  the  re- 
moval of  undesirable  constituents  (as  in  the 
conversion  of  pig  iron  into  steel)  or  the  addi- 
tion of  desirable  constituents  (as  in  the  case- 


PRACTICAL  APPLICATIONS  49 

hardening  and  the  manufacturing  of  "  chrome 
steel/'  " nickel  steel/7  etc.).  For  example, 
P.  Putz  has  shown  (J.  S.  C.  L,  1907,  614)  that 
the  predominant  effect  of  vanadium  in  steel  is 
to  decrease  the  size  of  the  ferrite  grains  and 
make  the  material  tougher;  it  renders  the 
ordinary  structure  due  to  pearlite  fine-grained 
and  homogeneous  (see  also  Beilby,  Proc.  Roy. 
Soc.  A.,  79,  463;  J.  S.  C.  L,  1907,  926). 

Now,  while  the  question  is  one  of  very  great 
complexity,  many  of  the  facts  at  present 
available  seem  to  indicate  that  one  of  the 
causes  favoring  the  fine-grained  structure  is 
the  inhibition  of  crystallization  by  substances 
colloidally  dissolved  in  the  molten  mass.  Thus 
part  of  the  carbon  in  iron  and  steel  exists  in  the 
graphitic  form,  and  as  graphite  is  slightly  soluble 
in  iron  (see  C.  Benedicks,  Metallurgie,  1908, 
5,  41;  J.  S.  C.  L,  1908,  406);  some  of  it  will, 
under  proper  conditions,  be  found  in  colloidal 
form  (Carnegie  Research  Reports,  J.  S.  C.  L, 
1908,  27,  570;  F.  Wust,  J.  S.  C.  L,  1907,  26, 
412;  Hersey,  J.  S.  C.  L,  27,  531).  Besides 
metals  may  dissolve  each  other  and  other 
substances  colloidally,  but  in  the  case  of  ordi- 
nary metals  this  is  not  easy  to  demonstrate. 


50  COLLOID  CHEMISTRY 

An  observation  recently  made  by  J.  Alex- 
ander *  is  of  interest  here.  Moissan  (Comptes 
rend.,  144,  593,  J.  S.  C.  L,  1907,  413)  has  noted 
that  the  addition  of  a  little  platinum  to  me- 
tallic mercury  causes  the  latter  to  "  emulsify  " 
in  water.  Upon  making  up  such  an  "  emul- 
sion," Alexander  noticed  that  the  supernatant 
fluid  remained  turbid  upon  standing,  and 
therefore  examined  the  fluid  in  the  ultrami- 
croscope,  which  revealed  the  presence  of  col- 
loidal metallic  particles  in  active  motion. 

DYEING.  —  The  difference  between  a  physi- 
cal mixture  and  a  chemical  compound  is 
frequently  illustrated  by  dissolving  out  the 
sulphur  from  a  mixture  of  iron  filings  and 
sulphur  dust,  and  showing  that  the  solvent, 
carbon  bisulphid,  does  not  affect  the  compound, 
ferrous  sulphid.  That  in  many  cases  dyeing 
is  due,  not  to  chemical  combination,  but  to  an 
adsorption  f  of  the  dye  by  the  colloidal  fiber,  is 
evident  from  the  fact  that  some  dyestuffs  can 
be  extracted  from  the  dyed  fiber  by  means  of 
alcohol.  Investigation  has  shown  that  many 

*  J.  S.  C.  I.,  1909,  28,  280. 

t  In  some  cases  adsorption  may  be  followed  by  undoubted 
chemical  combination. 


PRACTICAL  APPLICATIONS  51 

dyes  are  colloidal  in  solution,  and  the  selective 
coloring  of  various  fibers,  tissues,  cells,  nuclei, 
etc.,  is  probably  due  to  selective  adsorption  or 
precipitation  of  one  colloid  by  another.  The 
ultramicroscopic  researches  of  N.  Gaidukov 
(Zeitsch.  f.  angew  Chem.,  21,  393)  support  this 
view. 

The  phenomena  of  dyeing  are  rather  numer- 
ous and  complicated,  for  the  dyestuffs  are 
numbered  by  thousands,  and  the  various  fibers, 
tissues,  etc.,  such  as  cotton,  silk,  wool,  linen, 
jute  and  straw,  all  react  characteristically.  In 
some  cases  the  colloid  fiber  adsorbs  the  dye,  as 
with  basic  colors  which  dye  silk  and  wool 
directly;  in  other  cases  there  is  necessary  a 
mordant  which  is  first  adsorbed  and  then  fixes 
the  color.  Certain  colors  mutually  precipitate 
each  other  and  may  in  fact  serve  as  mordants 
for  each  other,  e.g.,  methylen  blue  and  dianil 
blue  2  R.;  patent  blue  V  and  magenta. 

Colloid  chemistry  also  throws  much  light 
upon  many  obscure  points  in  the  practical  art 
of  dyeing.  It  is  possible  to  obtain  much  more 
level  colors  in  old  dye  liquors  than  in  fresh 
ones,  and  here  it  seems  that  colloidally  dis- 
solved substances  are  responsible,  exercising 


52  COLLOID  CHEMISTRY 

a  restraining  action  upon  the  absorption  of  the 
color.  The  addition  of  Glaubers'  salt  facili- 
tates level  dyeing,  probably  by  its  action  as  an 
electrolyte,  producing  a  partial  coagulation  of 
the  dyestuff,  so  that  the  particles  of  the  latter, 
thereby  made  larger,  are  absorbed  more  slowly 
and  evenly. 

SOAP.  —  In  a  comprehensive  paper  entitled 
"  Modern  views  on  the  constitution  of  soap  " 
(see  J.  S.  C.  L,  1907,  26,  590)  Lewkowitsch 
epitomizes  the  views  of  Merklen  substantially 
as  follows:  " Commercial  soap  is  a  product 
having  an  essentially  variable  composition 
dependent  upon  (1)  the  nature  of  the  fatty 
acids,  (2)  the  composition  of  the  'nigre'  (in 
the  case  of  settled  soaps),  (3)  the  tempera- 
ture at  which  the  boiling  is  conducted;  it 
behaves  like  a  colloid  and  should  not  be  re- 
garded as  a  compound  of  sodium  salts  of  fatty 
acids,  with  which  a  definite  amount  of  water  is 
combined  chemically,  but  rather  as  an  'ab- 
sorption-product' whose  composition  is  a  func- 
tion of  the  environment  in  which  the  salts  of 
the  fatty  acids  happen  to  be  at  the  moment  of 
the  finishing  operations. " 

Merklen's  views  conflict  with  the  views  as  to 


PRACTICAL  APPLICATIONS  53 

the  chemical  composition  of  soap  previously 
advanced  by  Lewkowitsch,  who  states,  in 
conclusion:  "But  whatever  may  be  the  out- 
come of  renewed  experiments,  Merklen's  views 
cannot  fail  to  stimulate  further  research  into 
the  composition  of  soap,  and  thus  help  to  raise 
the  industry  of  soap-making,  which  has  too 
long  been  looked  upon  as  a  mere  art,  to  the 
rank  of  a  scientifically  well-founded  industry, 
the  operations  of  which  are  governed  by  the 
laws  of  mass  action,  the  phase  rule  and  the 
modern  chemistry  of  colloids. " 

The  colloidal  nature  of  soap  solutions  is 
indicated  by  their  turbidity  and  their  gelatin- 
ization.  That  the  detergent  action  of  soap  is 
consequent  upon  its  deflocculating  effect  was 
brought  out  in  the  interesting  Cantor  Lecture 
of  H.  Jackson  (J.  Soc.  Arts,  55,  1101  et  seq.), 
who  examined  microscopically  the  supernatant 
fluid  resulting  from  washing  a  dirty  cloth  with 
soap  and  water,  and  found  in  it  countless 
particles  in  a  state  of  oscillatory  motion 
("pedesis  ")•  When  an  individual  fiber  was 
bathed  in  soap  solution,  the  dirt  particles 
gradually  loosened  and  began  to  oscillate; 
upon  substituting  salt  solution  for  the  soap, 


54  COLLOID  CHEMISTRY 

the  particles  flocculated  and  the  motion 
ceased.  An  ultramicroscopic  examination  of 
the  detergent  effects  produced  by  soap  should 
prove  of  interest. 

In  this  connection  mention  must  be  made 
of  the  excellent  paper  of  W.  D.  Richardson 
on  "Transparent  Soap"  (J.  Amer.  Chem. 
Soc.,  30,  414),  which  he  terms  a  supercooled 
or  supersaturated  solution,  having  distinctly 
crystalline  tendencies  and  exhibiting  colloidal 
properties.  Having  in  mind  the  fact  that 
the  salts  of  the  higher  fatty  acids  dissolve 
in  water  as  colloids,  and  in  alcohol  as  crystal- 
loids (S.  Ya.  Levites,  Zeits.  Chem.  Ind.  Kol- 
loide,  2,  208,  et  seq.,  J.  S.  C.  L,  1908,  1134; 
Mayer,  Schaeffer,  and  Terroine,  Compt.  rend., 
146,  484)  and  also  the  fact  that  the  alcohol  or 
equivalent  solvents  (glycerol,  sugar,  etc.)  are 
used  in  transparent  soap,  it  seems  probable 
that  the  crystals  which  frequently  form  in  it 
are  due  to  the  slow  separation  of  such  part  of 
the  soap  as  is  in  crystalloid  solution.  This 
view  is  supported  by  the  fact  adduced  by 
Richardson  (loc.  cit.,  p.  418)  that  the  fatty 
acids  separated  from  the  crystals  had  a  higher 
melting  point  than  those  separated  from  the 


PRACTICAL  APPLICATIONS  55 

clear  matrix.  The  isolation  of  the  crystals 
was  difficult  because  of  their  ramifying  tend- 
ency, which  recalls  some  of  the  crystal  figures 
exhibited  by  some  mixtures  of  crystalloids  and 
colloids.  What  may  be  called  the  crystalloid 
phase  of  soap  is  apparently  governed  by  the 
same  factors  as  those  which  Tamman  has 
pointed  out  as  governing  the  crystallization  of 
supercooled  solutions,  i.e.,  1st,  the  specific  power 
of  crystallization;  2nd,  the  speed  of  crystalliza- 
tion; 3rd,  the  viscosity  (see  Zsigmondy,  Colloids 
and  the  Ultramicroscope,  p.  128  et  seq.).  Thus, 
gold  ruby  glass  when  quickly  cooled  (or  super- 
cooled) is  colorless,  but  acquires  a  red  color 
upon  reheating  to  the  softening  point.  By 
ultramicroscopic  examination  Zsigmondy 
showed  that  the  nuclei  of  metallic  gold,  which 
in  the  colorless  glass  were  amicroscopic,  grew 
into  ultramicroscopic  visibility  in  the  red  glass. 
It  therefore  seemed  to  the  author  that  a  most 
important  factor  in  determining  the  trans- 
parency of  transparent  soap  would  be  the 
speed  of  cooling,  and  some  experiments  were 
made  along  this  line. 

A  piece  of  commercial  transparent  soap  was 
melted  and  cast  into  two  cups,  one  of  which 


56  COLLOID  CHEMISTRY 

was  quickly  chilled  in  ice,  while  the  other  was 
allowed  to  cool  slowly  by  immersion  in  hot 
water.  The  quickly  cooled  piece  was  trans- 
parent, while  the  other  was  practically  opaque, 
and  showed  upon  ultramicroscopic  examina- 
tion much  larger  ultramicrons  than  the  trans- 
parent piece. 

After  standing  three  or  four  months,  the 
quickly  cooled  soap  was  still  transparent  to 
the  naked  eye,  whereas  large  opaque  spots 
could  be  seen  in  the  slowly  cooled  piece.  In 
the  ultramicroscope  the  former  appeared  as 
before,  whereas  the  latter  showed  large  and 
perfectly  resolvable  crystals  in  a  clear  matrix. 

These  experiments  give  us  an  inkling  as  to 
what  occurs  during  the  "  heat  treatment  "  and 
tempering  of  metals,  and  it  is  to  be  hoped  that 
some  technique  may  be  devised  that  will  give 
us  even  a  clearer  insight  than  does  "  etching," 
into  the  changes  that  occur  in  metals  in  metal- 
lurgical operations  (heat  treatment),  use,  age, 
and  even  "  disease  "  (tin  for  example). 

MILK.  —  From  a  colloid  chemical  stand- 
point, the  main  constituents  of  milk  may  be 
classified  as  follows: 


PRACTICAL  APPLICATIONS 


57 


In  crystalloid 
dispersion 


salts  (such  as  NaCl,  etc.) 
sugar  (lactose). 


In  colloidal       C  casein  —  an  unstable  or  irreversible  colloid. 
dispersion      (  lactalbumin  —  a  stable  or  reversible  colloid. 
In  suspension*     milk  fat. 

Most  formulas  and  recipes  for  modifying 
cows'  milk  for  infant  feeding,  and  for  that 
matter,  many  analyses,  combine  the  percent- 
ages of  lactalbumin  and  of  casein  under  the 
collective  title  of  "  total  proteids,"  thereby 
obscuring  the  highly  important  fact  that  the 
lactalbumin  stabilizes  and  protects  the  casein 
from  coagulation  by  acid  and  rennin.f 

The  subjoined  table  will  show  how  milks 
are  influenced  by  a  difference  in  the  ratio 
between  the  casein  and  lactalbumin. 

AVERAGE  COMPOSITION 


Lact- 

Kind  of  milk. 

Casein. 

albu- 

Fat. 

Behavior  with 
acid. 

Behavior  with 
rennin. 

min. 

Cow 

3  02 

0  53 

3  64 

Readily  coag- 

Readily 

ulates. 

coagulates. 

Woman  

1.03 

1  26 

3  78 

Not  readily 

Not  readily 

coagulated. 

coagulated. 

Ass 

0  67 

1  55 

1  64 

*  It  is  probable  that  some  of  the  fat  is  in  colloidal  dispersion, 
t  See  Alexander  and  Bullowa,  Jour.  Am.  Med.  Assoc.,  Vol. 
LV,  p.  1196   (Oct.  1,  1910). 


58  COLLOID  CHEMISTRY 

It  is  interesting  to  note  that  the  milks  in  the 
above  table  are  arranged  in  order  of  their 
digestibility,  which  also  corresponds  with  their 
relative  colloidal  protection.  Thus  Jacobi  has 
stated  that  asses'  milk  has  always  been 
recognized  as  a  refuge  in  digestive  disorders  in 
which  neither  mother's  milk  nor  cow's  milk  or 
mixtures  were  tolerated. 

The  addition  of  protective  colloids  to  cows' 
milk  stabilizes  it  and  makes  it  act  more  like 
mother's  milk  when  treated  with  acid  and 
rennin.  In  fact,  if  sufficient  protective  colloid 
be  added,  coagulation  of  the  casein  in  the 
stomach  may  be  entirely  prevented,  or  at  least 
the  coagula  kept  in  a  very  fine  state  of  sub- 
division. 

The  action  of  protective  colloids  is  beauti- 
fully illustrated  in  the  ultramicroscope,  which 
enables  us  to  see  the  individual  particles  of 
cows'  casein  in  active  motion  and  watch  the 
course  of  their  coagulation  by  acid,  first  into 
small  and  then  into  larger  and  larger  groups, 
whose  motion  decreases  as  their  size  increases, 
until  finally  they  sink  out  of  solution  in  coagu- 
lated masses.  If,  however,  some  gelatin  or 
gum  arabic  solution  be  added  to  the  cows' 


PRACTICAL  APPLICATIONS  59 

milk  before  the  addition  of  the  acid,  the  casein 
particles  continue  their  active  dance  and  do 
not  coagulate.  In  this  connection  it  is  in- 
teresting to  note  that  the  casein  particles  in 
mother's  milk  appear  to  be  much  smaller  than 
those  in  cow's  milk,  probably  because  of  the 
more  highly  protective  medium  in  which  they 
are  formed  and  exist. 

Although  their  method  of  action  was  not 
perfectly  understood,  protective  colloidal  sub- 
stances have  for  years  been  used  in  the  modi- 
fication of  cow's  milk  for  infants.  For  over 
thirty  years  Jacobi  has  advocated  the  addi- 
tion of  gelatin  and  gum  arabic  to  cow's  milk 
and  infant's  diet,  and  the  use  of  gruels,  dex- 
trinized  starch  and  similar  reversible  colloids 
is  familiar  to  all.  It  is  interesting  to  note  that 
sodium  citrate,  which  is  largely  employed  as 
an  addition  to  cow's  milk,  acts  as  a  protective 
colloid,  and  when  going  into  solution  actually 
exhibits  actively  moving  ultramicrons  in  the 
ultramicroscope,  a  fact  which  indicates  its 
colloidal  condition. 

In  addition  to  stabilizing  the  casein,  pro- 
tective colloids  in  milk  have  a  very  important 
influence  on  the  milk  fat.  In  the  first  place 


60  COLLOID  CHEMISTRY 

is  to  be  considered  the  emulsifying  and 
emulsostatic  action  of  reversible  colloids.  Of 
much  greater  importance,  however,  is  the 
result  of  stabilizing  the  casein,  for  insufficiently 
protected  casein  in  curding  carries  down 
mechanically  most  of  the  milk  fat  present, 
yielding  a  greasy,  fatty  curd  which  is  very 
difficult  for  the  digestive  juices  to  dissolve. 

ICE  CREAM.  —  It  is  a  fact  well  known  to 
practical  ice  cream  makers,  and  amply  proven 
by  experience,  that  ice  cream  made  without 
eggs,  gelatin  or  some  similar  colloidal  ingredi- 
ent, is  gritty,  grainy  or  sandy,  or  else  soon 
becomes  so  upon  standing;  whereas  ice  cream 
made  with  small  quantities  of  colloids  possesses 
that  rich,  mellow,  velvety  texture  so  much  in 
demand.  Here  the  added  colloid  acts  as  an 
inhibitor  of  crystallization  or  practically  speak- 
ing as  a  preserver  of  texture.  The  added 
colloid,  especially  gelatin,  which  is  the  one 
most  frequently  used,  also  serves  as  a  protective 
colloid  in  preventing  the  coagulation  of  casein, 
apparently  an  irreversible  hydrosol  and  a 
normal  constituent  of  ice  cream.  In  view  of 
what  has  been  said  above,  it  is  evident  that 
gelatin  thus  renders  ice  cream  more  digestible. 


PRACTICAL  APPLICATIONS  61 

A  very  misleading  impression  is  given  by 
some  official  food  chemists  referring  to  gelatin 
in  ice  cream  as  a  " filler,"  which  naturally  leads 
to  the  idea  that  it  is  an  inferior  ingredient 
added  in  quantity  to  cheapen  the  product. 
But  as  gelatin  is  expensive  and  as  but  |  per 
cent  is  used,  such  a  view  is  evidently  erroneous. 
The  food  value  of  gelatin  as  a  protector  of  the 
body's  nitrogen  being  generally  admitted,  and 
its  effect  in  milk  being  very  beneficial  from  a 
digestive  point  of  view,  its  use  in  ice  cream  in 
the  quantities  referred  to  is  necessary,  legiti- 
mate and  scientific. 

CONFECTIONARY.  —  In  gum  drops,  marsh- 
mallows,  "  moonshine  "  and  other  candies,  use 
is  made  of  gum  arabic,  gelatin,  albumen,  and 
other  colloids  to  prevent  the  crystallization  of 
the  sugar.  Thus,  besides  adding  to  the  food 
value,  they  give  the  candy  a  smooth  and 
agreeable  taste,  and  preserve  it  in  saleable 
condition. 

BREWING.  —  Beer  contains  dextrin  and  al- 
bumin, both  colloids.  In  the  brewing  process 
many  factors  appear  which  tend  to  coagulate 
the  albumen.  The  influence  of  solid  surfaces 
is  illustrated  by  changing  the  walls  of  the 


62  COLLOID  CHEMISTRY 

fermenting  vessel.  Thus  a  certain  wort  fer- 
mented in  glass  or  enameled  vessels  showed 
0.2450  per  cent  of  albumen;  the  same  wort 
fermented  in  a  paraffin-lined  vessel  showed 
0.1925,  and  in  a  vessel  lined  with  pitch  only 
0.1750  per  cent  of  albumen.  Old-fashioned 
brewers  would  never  use  any  vessel  unless  it 
had  first  been  treated  with  a  decoction  of 
malt  kernels  and  nut  leaves,  or  else  with 
"fassgelager  "  (barrel  dregs)  which  acts  like 
the  so-called  "bierstein,"  a  deposit  consisting 
chiefly  of  organic  substances  that  forms  upon 
new  surfaces  and  protects  albumen  from 
coagulation  by  their  influences. 

The  influence  of  fluid  surfaces  is  evident 
from  the  fact  that  in  the  chemical  analysis  of 
beer,  benzine,  benzol,  chloroform,  etc.,  may 
be  used  to  coagulate  and  shake  out  the  beer 
colloids. 

The  formation  of  gas  bubbles  tends  to 
coagulate  the  dissolved  albumen,  and  this  fact 
killed  the  so-called  "  Vacuum  Fermentation 
Process."  The  jarring  due  to  transportation 
or  even  to  passing  trains  may  have  a  deleteri- 
ous effect.  A  slight  trace  of  acid  tends  to 
stabilize  the  albumen  as  do  the  tannin  and 


PRACTICAL  APPLICATIONS  63 

resins  from  the  hops,  the  dextrins  from  the 
mash  and  the  inorganic  colloids  of  calcium  and 
magnesium.  A  proper  balance  between  the 
dextrin  and  albumen  is  necessary  for  the 
formation  of  a  lasting  foam  and  a  desirable 
"body"  (Vollmundigkeit). 

In  America  where  beer  is  served  icy  cold, 
the  chilling  produces  cloudiness,  consequent 
upon  a  coagulation  of  albumen.  This  was 
cleverly  overcome  by  Wallerstein,  who  in- 
troduced a  proteolytic  enzyme  which  increases 
the  degree  of  dispersion  of  the  albumen  and 
thus  prevents  the  clouding. 

TANNING.  —  The  skins  of  animals  (hide) 
constitute  an  organized  colloid  jelly,  formed  of 
bundles  of  fine  fibrils,  about  1  /*  in  diameter, 
bound  together  by  a  cementing  material  of 
similar  chemical  composition,  which  is  largely 
removed  by  the  liming  and  other  treatment, 
preceding  the  tannage  proper. 

When  the  swollen  hide  is  placed  in  the  acid 
tannin  solution*  (tan  liquor),  the  tannin  is 
powerfully  adsorbed  by  the  fiber  and  combines 
with  it  to  form  leather.  It  is  still  a  moot 

*  In  alkaline  solution  both  the  tannin  and  thejaide  are 
negatively  charged  and  no  tanning  occurs. 


64  COLLOID  CHEMISTRY 

question  whether  the  combination  is  "phys- 
ical "  or  "chemical,"  but  since  the  fixation  of 
the  tannin  follows  an  adsorption  isotherm  and 
is  reversible  in  the  presence  of  alkalis,  it  may 
justly  be  called  a  " colloid  combination" 
which  partakes  of  the  nature  of  both.  The 
positively  charged  hide  and  the  negatively 
charged  tannin  mutually  coagulate  each  other. 
Gelatin  when  neutral  and  free  from  electro- 
lytes does  not  precipitate  pure  tannin,  but  in 
acid  solution  it  takes  a  positive  charge  and  is 
tanned.  The  tanning  process  may  be  aided 
electrically  by  giving  the  hide  a  suitable 
potential,  positive  in  the  case  of  tannin  and 
negative  in  the  case  of  chromium  compounds. 

RUBBER.  —  Rubber  is  made  by  coagulating 
the  milky  juice  (latex)  of  various  plants. 
Rubber  latices  are  emulsions  stabilized  by 
protective  colloids  (proteins  or  peptones)  and 
the  nature  of  the  coagulant  depends  upon  the 
nature  of  the  protector.  Thus,  formaldehyde 
preserves  latices  whose  protectors  are  proteins, 
but  coagulates  Kickexia  latex  by  precipitating 
the  protective  peptones. 

Vulcanization  consists  of  the  combination 
of  sulphur  with  rubber.  At  first  the  sulphur 


PRACTICAL  APPLICATIONS  65 

is  adsorbed;  and  then  by  heating,  part  of  it 
enters  into  a  close  combination,  probably  true 
chemical  combination. 

PHOTOGRAPHY.  —  The  photographic  plate 
owes  its  sensitiveness  to  an  " emulsion"  of 
colloidal  silver  halides  stabilized  by  a  protec- 
tive colloid  (gelatin,  albumen  or  collodion). 
The  degree  of  dispersion  is  controlled  by  the 
conditions  of  precipitation  of  the  silver  salt 
and  the  subsequent  treatment  of  the  emulsion 
(ripening).  The  latent  image  formed  upon 
the  exposure  of  the  plate  to  light  is  probably 
an  adsorption  compound  between  colloidal 
silver  and  the  silver  halides. 

BOILER  SCALE.  —  In  addition  to  containing 
various  salts  intended  to  precipitate  scale- 
forming  ingredients,  most  formulas  for  "  boiler- 
compounds  "  and  scale-preventing  mixtures 
include  such  substances  as  glue,  dextrin, 
starch,  potatoes,  tannin,  extract  of  hemlock, 
etc.  These  colloids  undoubtedly  prevent  the 
formation  of  hard  crystalline  scale,  either  by 
inhibiting  to  some  extent  the  precipitation  of 
the  scale-forming  salts  or  by  keeping  the 
precipitate  in  an  extremely  fine  non-crystal- 
line condition. 


66 


COLLOID  CHEMISTRY 


CEMENT,  MORTAR  AND  PLASTER.  —  When 
freshly  mixed,  cement  and  mortar  contain 
colloidal  sols  or  gels,  which  gradually  coagu- 
late or  "set  "  and  bind  the  crystalline  elements 
of  the  plaster  into  a  coherent  whole. 

The  setting  of  the  plaster  of  Paris  is  delayed 
by  glues,  gums  and  other  colloidal  substances, 
and  "retarders  "  of  this  character  have  been 
in  use  for  years.  On  preparing  some  micro- 
scope slides  with  a  mixture  containing  equal 
parts  of  plaster  of  Paris  and  water,  to  which 
had  been  added  varying  proportions  of  gelatin, 
the  following  results  were  observed: 


Per  cent 
gelatin. 

Time  to  set 
in  minutes. 

Microscopic  appearance  of  slide. 

0 

40 

Characteristic  interlacing  crystals  of  calcium 

sulphate. 

T&« 

50 

No  true  crystals  except  in  a  few  spots,  where 

some  colloid-free  solution  had  diffused  out. 

Elsewhere  aborted  sphero-crystals. 

& 

260 

No  true  crystals. 

1 

910 

No  true  crystals. 

* 

960 

No  true  crystals. 

1 

Not  set  in 

No  true  crystals. 

48  hours. 

2 

Not  set  in 

No  true  crystals. 

48  hours. 

FILTRATION.  —  Successful  filtration  depends 
upon  the  use  of  a  septum  or  filtering  medium, 


PRACTICAL  APPLICATIONS  67 

whose  pores  or  orifices  are  small  enough  to 
hold  back  the  particles  it  is  desired  to  sepa- 
rate from  the  fluid;  or  the  pores  may  be- 
come small  enough  by  the  deposit  upon  or  in 
them  of  some  of  the  precipitate,  or  of  some 
added  material,  such  as  paper  pulp,  kieselguhr 
or  shredded  asbestos.  It  is,  therefore,  evident 
that  the  presence  of  protective  colloids,  by 
tending  to  produce  the  finely  dispersed  or 
"hydrosol"  condition  of  the  particles,  favors 
their  passage  through  the  filter.  Thus  a  gold 
hydrosol  with  particles  of  20-30  ju^  and  con- 
taining albumen,  passed  freely  through  a 
Pukall  and  a  Maassen  filter.  In  the  absence 
of  the  protective  albumen,  the  colloidal  gold 
was  adsorbed  by  the  filter,  gradually  clogging 
the  pores  until  the  filtrate,  at  first  red,  became 
colorless.  In  technical  practice,  wherever 
possible,  a  coagulated  precipitate  is  formed, 
whose  large  particles  are  held  back  with  com- 
parative ease.  It  is  very  difficult  to  filter  glue 
or  gelatin  solutions  or  precipitates  formed  in 
the  presence  of  protective  colloids. 

The  successful  treatment  of  sewage,  back- 
waters and  trade  effluents  depends  largely  upon 
the  separation  from  them  of  colloidal  impuri- 


68  COLLOID  CHEMISTRY 

ties  by  coagulation,  adsorption  and  filtration. 
The  old  ABC  method  depended  upon  the  use 
of  alum,  blood  and  clay  (whence  the  name)  to 
make  a  coagulum  which  would  carry  down 
suspended  matter.  Ferrous  sulphate  and  lime 
(yielding  a  coagulum  of  ferric  hydroxid)  and 
alum  are  also  used  as  clarifiers  and  coagulants. 
Filtration  through  sand,  coke,  etc.,  is  made  use 
of  to  adsorb  finely  dispersed  impurities. 

Animal  charcoal  and  fuller's  earth  decolorize 
sugar  and  oils  respectively,  because  of  their 
powerful  adsorptive  action. 

CHEMICAL  ANALYSIS.  —  The  presence  of  col- 
loids, especially  in  technical  products  or  solu- 
tions, may  lead  to  grave  errors  in  analysis,  so 
that  the  chemist  should  destroy  them  by 
ignition,  or  else  nullify  their  effects  by  the 
addition  of  a  sufficient  excess  of  coagulant  or 
precipitant.  Reversible  colloids  which  are 
frequently  referred  to  under  the  vague  term 
" organic  matter  "  may  act:  (1)  by  totally  or 
partially  preventing  the  formation  of  precipi- 
tates, just  as  tartaric  acid  and  tartrates  prevent 
the  precipitation  of  alumina,  chromic  oxid,  and 
ferric  oxid  (see  Yoshimoto,  J.  S.  C.  I.,  1908, 
27,  952);  (2)  by  preventing  the  satisfactory 


PRACTICAL  APPLICATIONS  69 

filtration  of  the  precipitate  formed  (see  Mooers 
and  Hampton,  J.  Am.  Chem.  Soc.,  30,  805);  (3) 
by  rendering  precipitates  difficult  to  wash  and 
purify  (see  Duclaux,  J.  S.  C.  L,  1906,  25,  866). 

A  few  experiments  will  serve  to  make  clear 
the  importance  of  these  remarks.  Three 
solutions  of  lead  acetate  were  taken;  to  the 
first  was  added  hydrochloric  acid  which  yielded 
a  heavy  coagulated  precipitate;  to  the  second 
was  added  sodium  chlorid  (a  less  highly 
ionized  precipitant)  which  yielded  a  colloidal 
precipitate  of  lead  chlorid;  to  the  third  was 
added,  first,  a  little  glue  solution  and  then 
sodium  chlorid  which  in  this  case  gave  no 
precipitate  at  all. 

Again  in  the  presence  of  glue,  silver  nitrate 
gives  with  sodium  chlorid  only  an  opalescence 
which  passes  through  filter  paper.  Even  a 
large  excess  of  hydrochloric  acid  fails  to  pro- 
duce a  precipitate.  But  upon  adding  silver  ni- 
trate solution  to  a  chlorid  solution  containing  no 
colloid,  a  copious  precipitation  occurs  at  once. 

PHARMACY.  —  Colloids,  such  as  gum  arabic, 
Irish  moss,  tragacanth,  etc.,  are  largely  used 
in  pharmacy  in  the  preparation  of  emulsions. 
If  ferric  chlorid  be  added  to  gum  arabic  emul- 


70  COLLOID  CHEMISTRY 

sion  of  cod  liver  oil,  it  coagulates  the  gum,  and 
the  oil,  no  longer  protected  by  the  emulso- 
static  action  of  the  gum,  promptly  separates 
out. 

Colloidal  silver  (collargol,  argyrol),  colloidal 
mercury  (hygrol,  blue  ointment),  and  col- 
loidal sulphur  (ichythol)  are  largely  used 
medicinely.  Ferric  salts,  especially  the  chlo- 
rid  which  readily  hydrolyzes  into  the  hydrate, 
act  as  styptics  or  hemostatics  by  coagulating 
the  blood  colloids.  The  action  of  disinfectants 
is  largely  controlled  by  colloid-chemical  factors 
—  the  disinfectants  are  adsorbed  by  bacteria, 
and  either  coagulate  their  protoplasm  or  flock 
them  out. 

FOODS  AND  THEIR  PREPARATION.  —  It  is  a 
serious  error  to  judge  foods  upon  the  basis  of 
a  bald  chemical  or  calorific  analysis.  Fat, 
protein,  carbohydrate  and  calories  are  not 
alone  the  criteria  of  food  value  —  the  physical 
condition  of  food  largely  governs  its  useful- 
ness to  the  organism.  The  experiences  of 
centuries  has  taught  us  the  value  of  "light" 
bread  or  cake,  leavened  by  yeast  or  baking 
powder  until  it  presents  an  enormous  surface 
to  the  digestive  juices;  unleavened  bread  was 


PRACTICAL  APPLICATIONS  71 

eaten  only  in  time  of  stress,  as  we  learn  from 
the  Bible.  The  meats  yielded  by  young 
animals  are  more  juicy  and  tender  than  those 
obtained  from  older  animals,  because  the  latter 
are  formed  from  tissues  partially  dehydrated 
by  age. 

The  ancient  art  of  cooking  involves  many 
factors  besides  mere  digestibility  and  assimila- 
tion; taste,  flavor,  odor  and  variety  are 
important.  Egg  albumen  when  cooked  is 
probably  more  slowly  absorbed  and  loses  its 
species-specificity;  therefore,  some  people  who 
have  an  idiosyncracy  against  raw  eggs  can 
eat  cooked  eggs.  Cream  is  an  emulsion  of  fat 
in  an  aqueous  medium  and  wets  paper;  butter 
is  an  emulsion  of  water  in  a  fatty  medium  and 
greases  paper. 

PHYSIOLOGY  AND  PATHOLOGY. — The  changes 
which  occur  on  almost  all  physiological  proc- 
esses are  remarkable  not  only  because  of 
their  very  profound  nature,  but  also  because 
they  are  produced  at  comparatively  low  tem- 
peratures and  in  the  presence  of  very  dilute 
reagents.  The  living  organism  disintegrates 
proteins,  oxidizes  carbohydrates  and  with  the 
same  apparent  ease  synthesizes  substances  of 


72  COLLOID  CHEMISTRY 

great  complexity.  Powerful  reagents  and  high 
temperatures,  which  would  be  destructive  to 
life,  are  necessary  to  bring  about  changes  of 
this  character  under  ordinary  laboratory  con- 
ditions. 

The  body  and  plant  colloids  (biocolloids) 
consist  of  carbohydrates  (starch,  cellulose, 
glycogen),  proteins  (plant  and  animal  al- 
bumins), and  lipoids  (lecithin,  cholesterin,  fats 
and  oils).  Each  tissue  has  a  normal  turgor  or 
state  of  swelling  which  is  greatly  influenced 
by  acids,  alkalis  and  salts.  The  swelling  and 
shrinking  of  tissues,  together  with  their  selective 
adsorption  and  the  differential  diffusion  of 
solutions  through  them,  account  for  or  accom- 
pany many  physiological  phenomena,  both 
normal  and  pathological.  Thus,  fibrin  and 
gelatin  swell  much  more  in  very  dilute  acid 
than  in  distilled  water,  but  the  swelling  is 
depressed  by  salts.  Fibrin  is  so  sensitive  that 
it  swells  in  the  presence  of  traces  of  acid  quite 
undetectable  by  ordinary  indicators,  such  as 
litmus;  in  fact  fibrin  itself  is  a  most  sensitive 
indicator.* 

*  Though  the  normal  H  ion  concentration  of  the  blood  is 
0.37  X  10~7,  a  concentration  of  1.00  X  10~7  nH  represents 
an  advanced  acid  intoxication. 


PRACTICAL  APPLICATIONS  73 

Local  accumulation  of  acid  in  the  organism 
may  cause  swelling  (edema) ;  for  example,  in- 
sect stings,  which  may  be  imitated  by  stinging 
gelatin  with  a  needle  dipped  in  acid.  If  acid 
accumulates  in  an  organ  with  a  rigid  capsule 
(eye  or  kidney),  the  swelling  tends  to  establish 
a  vicious  circle  (glaucoma,  nephritis)  by  com- 
pressing the  blood  vessels  and  cutting  down 
the  alkaline  blood  stream,  which  is  unable  to 
wash  out  the  acids  (mainly  CO2)  formed  by 
living  protoplasm. 

If  the  oxidation  processes  of  the  body  are 
normal,  the  hydrogen  in  foods  is  oxidized 
mainly  to  water  and  the  carbon  mainly  to 
carbonic  acid  —  a  gaseous  acid  which  is  ex- 
haled without  demanding  fixed  alkali  or  pro- 
tein of  the  organism  for  its  elimination.  It 
would  require  nearly  two  pounds  of  pure  caus- 
tic soda  to  neutralize  the  acidity  produced  daily 
by  an  average  man.  In  the  case  of  pathologi- 
cal oxidation,  however,  other  non-volatile  acids 
are  formed  and  a  condition  called  "acidosis  " 
may  arise,  which  is  in  reality  a  diminished 
alkalinity,  recognizable  by  the  fact  that  an 
abnormally  large  quantity  of  bicarbonate  of 
soda  is  needed  to  render  the  urine  alkaline. 


74  COLLOID  CHEMISTRY 

These  acids  may  cause  disturbances  of  the 
body  colloids,  disease  and  even  death.  In  fact, 
throughout  life  there  is  a  gradual  syneresis  of 
the  biocolloids  accompanied  by  visible  shrink- 
ing and  loss  of  water  —  compare  the  chubby 
hand  of  a  child  with  that  of  an  old  man.  In 
plants  an  analogous  process  occurs  in  lignifi- 
cation. 

DIGESTION.  —  The  digestive  process  is  pre- 
liminary to  the  actual  adsorption  and  use  of 
food  by  the  organism,  and  has  for  its  object 
the  modification  or  change  of  the  ingested  food 
into  such  forms  or  such  substances  as  may  be 
absorbed  in  the  lower  part  of  the  digestive 
tube.  To  have  a  correct  understanding  of  the 
absorption  of  the  products  of  digestion,  we 
must  bear  in  mind  the  fact  that  the  walls  of  the 
digestive  tract  act  as  semipermeable  colloid 
membranes  and  that  absorption  involves  dif- 
fusion into  or  through  these  membranes  or 
their  constituent  cells.  Substances  in  crystal- 
loidal  solution,  and  colloidal  sols  whose  par- 
ticles are  sufficiently  small,  represent  then  the 
two  classes  of  digestion  products  which  are 
diffusible  and  therefore  absorbable. 

Food  as  ingested  consists  mainly  of  sub- 


PRACTICAL  APPLICATIONS  75 

stances  that  may  be  grouped  into  two  classes: 

1.  Crystalloids  —  such    as    water,    sugars, 
sodium  chlorid,  etc. 

2.  Colloids  —  such  as  starch,  proteins,  emul- 
sions, etc. 

The  crystalloids  in  foods  are  usually  absorbed 
directly,  although  sucrose,  for  example,  under- 
goes inversion.  The  colloids,  as  a  rule,  are  not 
directly  absorbable,  and,  for  the  most  part, 
digestion  consists  in^the  disintegration  of  the 
colloidal  complexes  of  the  food,  so  that  they 
can  actually  diffuse  into  the  organism  and 
there  undergo  further  changes.  Colloidal  gels 
or  even  sols  whose  particles  are  of  large  size 
are,  practically  speaking,  non-diffusible,  and 
must,  therefore,  be  reduced  to  a  more  finely 
dispersed  state. 

Investigation  has  demonstrated  that  the 
high  efficiency  of  the  digestive  juices  is  mainly 
due  to  small  quantities  of  certain  colloidal 
substances  called  enzymes  (such  as  ptyalin, 
pepsin  and  pancreatin)  which  act  as  catalyzers, 
enormously  hastening  reactions  which  would 
otherwise  proceed  so  slowly  that,  practically 
speaking,  they  would  not  occur  at  all.  The 
enzymes  appear  to  act  by  forming  with  the 


76  COLLOID  CHEMISTRY 

substrate  a  combination  of  unstable  character, 
which  breaks  down  and  liberates  the  enzyme 
again  to  continue  the  operation.  Recently 
W.  M.  Bayliss,  in  his  interesting  monograph 
on  "The  Nature  of  Enzyme  Action,"  has 
shown  that  in  all  probability  "the  compound 
of  enzyme  and  substrate,  generally  regarded  as 
preliminary  to  action,  is  in  the  nature  of  a 
colloidal  adsorption  compound."  Anyone  who 
has  seen  in  the  ultramicroscope  the  extremely 
active  motion  of  the  individual  particles  in 
colloidal  solutions,  can  readily  imagine  the 
terrific  bombardment  a  substance  must  un- 
dergo when  a  colloid  enzyme  is  concentrated 
on  its  surface  by  adsorption,  and  indeed  it 
seems  probable  that  enzymes  actually  produce 
their  effects  by  virtue  of  their  specific  surface 
actions  and  the  motion  of  their  particles. 

In  order  to  find  out  if  this  idea  could  be 
verified  by  actual  observation,  the  author 
watched  under  the  ultramicroscope  the  action 
of  diastase  upon  potato  starch  grains  and  the 
action  of  pepsin  upon  coagulated  egg  albumen. 

In  the  first  case,  actively  moving  ultra- 
microns  in  the  diastase  solution  gradually 
accumulated  about  the  starch  grains,  which 


PRACTICAL  APPLICATIONS  77 

after  a  time  showed  a  ragged  and  gnawed 
margin.  While  the  adsorption  and  motion  of 
the  larger  ultramicrons  was  all  that  could  be 
followed,  the  bright  appearance  of  the  field 
indicated  that  more  numerous  finer  particles 
were  present,  and  some  apparently  of  inter- 
mediate size  were  seen. 

For  observations  on  albumen  there  was  used 
a  dilute  solution  of  white  of  an  egg  which  has 
been  heated  nearly  to  boiling.  It  was  opales- 
cent and  in  the  ultra  apparatus  exhibited  a  field 
full  of  bright  and  rapidly  moving  ultramicrons. 
Upon  allowing  a  droplet  of  essence  of  pepsin 
(Fairchild's,  containing  15  per  cent  of  alcohol 
by  weight)  to  diffuse  in,  an  immediate  coagu- 
lation occurred,  the  particles  clumping  into 
very  large  masses.  A  droplet  of  decinormal 
hydrochloric  acid  was  then  allowed  to  diffuse 
in,  whereupon  the  large  masses  broke  up  in 
small  groups  and  single  ultramicrons,  which 
once  more  resumed  their  original  motion. 
Soon,  however,  the  albumen  particles  began 
to  grow  smaller  and  disappear,  the  field  all  the 
while  becoming  brighter  and  brighter,  indicat- 
ing the  concommitant  appearance  of  smaller 
ultramicrons  or  amicrons.  In  vitro  the  addi- 


78  COLLOID  CHEMISTRY 

tion  of  the  pepsin  to  the  opalescent  albumen 
solution  caused  it  to  clear  gradually,  even  at 
room  temperature. 

Enzymes  are  inactivated  to  a  greater  or  less 
extent  by  shaking,  heating,  electrolytes,  etc., 
all  of  which,  as  is  well  known,  cause  the 
coagulation  of  colloidal  solutions  and  a  result- 
ing decrease  in  the  activity  of  the  motion  of 
their  constituent  particles.  Another  feature 
of  interest  is  that  the  action  of  enzymes  is 
reversible,  a  fact  that  does  not  come  much 
into  evidence  because  of  the  dilution  and 
removal  by  diffusion  of  the  products  formed. 
In  cells,  tissues  and  organs,  however,  changes 
of  concentration  again  occur  and  synthetic 
processes  may  result. 

One  principle  of  colloid  chemistry  is  of  the 
utmost  importance  in  digestion,  namely:  the 
protective  action  of  reversible  colloids,  which 
stabilize  or  protect  from  coagulation  irrevers- 
ible or  unstable  colloids.  Mucin  and  analo- 
gous colloidal  substances  undoubtedly  have  a 
function  of  this  character,  which  may  in  some 
cases  account  for  the  variance  between  the 
action  of  natural  and  artificial  digestive  juices. 
The  effects  of  colloidal  protection  are  in 


PRACTICAL  APPLICATIONS  79 

evidence  in  almost  all  physiological  reactions 
and  processes,  and  it  is  indeed  extremely 
doubtful  if  there  ever  occurs  in  vivo  any 
chemical  reaction  which  is  not  greatly  in- 
fluenced by  the  colloids  always  present. 

ABSORPTION,  SECRETION  AND  EXCRETION.  — 
These  are  largely  affected  by  the  swelling  and 
shrinking  of  the  body  colloids  and  by  selective 
adsorption  and  diff erential  diffusion.  It  must 
be  remembered  that  the  blood  is  in  reality  a 
circulating  fluid  colloid,  whose  attraction  for 
water  is  greater  in  the  "acid  "  or  venous  con- 
dition, than  it  is  in  the  "  alkaline  "  or  arterial 
condition.  Tissues  and  organs  well  supplied 
with  venous  blood  tend  to  adsorb  water 
(intestine);  whereas  those  well  supplied  with 
arterial  blood  tend  to  give  up  (secrete  or  ex- 
crete) water  (kidney);  and  as  the  blood  is 
passing  in  a  continuous  stream,  the  process 
continues  as  long  as  the  water  supply  permits 
and  until  the  blood  is  in  equilibrium  with  the 
other  tissues.* 

*  The  functioning  of  organs  is  largely  controlled  by  nervous 
influences.  Thus  a  sudden  nervous  shock  may  by  vaso-dilation 
send  an  excessive  supply  of  arterial  blood  through  the  mes- 
enteric  arteries  (an  "internal  blush"),  and  result  in  a  secre- 
tion of  fluid  into  the  intestine  (nervous  diarrhea). 


80  COLLOID  CHEMISTRY 

Conditions  which  decrease  the  capacity  of 
the  blood  and  tissues  to  hold  water  (diuretics, 
hyperglucemia  and  acidosis  in  diabetes)  natu- 
rally result  in  the  elimination  of  the  excess  or 
"free  "  water  (polyuria,  diarrhea). 

Minute  quantities  of  acid  increase  the  swell- 
ing capacity  of  colloids,  which  quickly  reaches 
a  maximum;  after  which  increasing  acidity 
causes  shrinking.  Neutral  salts  oppose  the  ac- 
tion of  acids  apparently  by  driving  back  the 
ionization  of  the  acid  and  thereby  reducing  the 
H-ion  concentration  which  is  the  controlling 
factor. 

The  action  of  selective  adsorption  and 
differential  diffusion  in  effecting  secretion  and 
excretion  must  be  at  once  manifest.  Easily 
hydrolyzable  compounds  may  be  thus  split  up 
in  the  body,  and  yield  secretions  of  acid  nature 
like  the  gastric  juice,  or  of  alkaline  nature  like 
the  pancreatic  juice,  depending  upon  the 
structure  of  the  organ,  the  location  of  its 
cavity  and  of  its  afferent  and  efferent  vessels. 
Individual  compounds  in  the  blood  stream  or 
other  body  juices  may  also  be  selectively 
diffused  out,  concentrated  or  separated  from 
other  accompanying  substances.  By  selective 


PRACTICAL  APPLICATIONS 


81 


adsorption,  circulating  substances  may  be 
fixed  and  taken  from  the  circulation;  in  fact, 
poisons  are  usually  taken  up  selectively  by 
certain  organs  and  tissues. 

An    insight    into    the   mechanism   of   body 
processes  may  be  obtained  by  considering  the 


Convoluted 
Tubule^ 


Vascular 
Plexus 
(Ohmerulus) 


f|l<~7iy/£  of  Renal  Artery 
FIG.  1.     Glomerular  structure.* 

functioning  of  the  kidney  (see  Fig.  1).  The 
Malpighian  tufts  are  plentifully  supplied 
with  arterial  blood  having  "free  water/'  and 

*  From  Dr.  J.  G.  M.  Bullowa's  translation  of  Bechhold's 
"Colloids  in  Biology  and  Medicine,"  D.  Van  Nostrand  Co., 
1919. 


82  COLLOID  CHEMISTRY 

under  the  pulsating  pressure*  of  the  blood 
stream,  they  ultrafilter  off  a  very  dilute  but 
copious  blood  ultra-filtrate  into  the  long  con- 
voluted tubules.  The  tubules,  however,  are 
plentifully  supplied  with  venous  blood,  which 
is  unsaturated  with  water  and  which  there- 
fore reabsorbs  most  of  the  water  together 
with  some  of  the  dissolved  substances  contained 
in  the  preliminary  excretion ;  so  that  there  drips 
into  the  pelvis  of  the  kidney  a  concentrated 
urine  having  in  solution  many  of  the  substances 
found  hi  the  blood,  but  in  a  much  higher 
concentration.  Bechhold  estimates  that  the 
average  of  two  liters  of  urine  voided  daily  by  an 
average  man,  represents  a  preliminary  excretion 
of  fifty  liters,  of  which  forty-eight  are  reab- 
sorbed  within  the  kidney  itself. 

In  plants,  differential  diffusion  and  selective 
adsorption  seem  to  be  intimately  bound  up 
with  growth  and  the  circulation  of  the  sap. 
The  plant  tissues  are  mainly  colloidal  gels  or 
finely  integrated  structures,  and  as  the  sap 
circulates  or  diffuses  through  them,  each  tissue 

*  Since  the  vas  defferens  has  a  smaller  lumen  than  the  vas 
efferens,  a  "  back  pressure  "  is  created  within  the  Malpighian 
tufts. 


PRACTICAL  APPLICATIONS  83 

selectively  adsorbs  and  elaborates  certain 
particular  constituents.  Thus  with  the  potato 
and  tapioca  plants  the  starch  forming  sub- 
stances are  fixed  in  the  roots;  with  the  sago 
palm  they  are  fixed  in  the  stem  pith ;  and  with 
cereal  grains,  in  the  seeds.  As  long  as  the 
adsorptive  tissues  are  unsaturated  or  are 
multiplied,  so  long  can  growth  continue,  the 
stem  and  branches  taking  up  the  substances 
required  for  the  upward  growth,  and  the  root 
taking  up  those  required  for  the  downward 
growth. 

When  we  consider  the  great  variety  of  bio- 
colloids  and  their  susceptibility  to  changes  of 
structure  and  diffusive  or  adsorptive  capacity, 
we  can  easily  understand  the  almost  infinite 
number  of  reactions  that  may  go  on  within 
their  recesses,  as  they  swing  the  balance  of  the 
law  of  mass  action  over  particles  reduced  to 
a  reactive  degree  of  subdivision. 


BIBLIOGRAPHY 


The  following  are  some  of  the  more  important  standards  of 
reference: 

ENGLISH 

H.  BECHHOLD,  "Colloids  in  Biology  and  Medicine"  (trans. 

by  Dr.  J.  G.  M.  Bullowa).     1919. 

E.  F.  BURTON,  "The  Physical  Properties  of  Colloidal  Solu- 
tions."    1916. 
M.  H.  FISCHER,   "(Edema  and  Nephritis."     1915.     "Fats 

and  Fatty  Degeneration."     1917. 
Wo.  OSTWALD,  "Theoretical  and  Applied  Colloid  Chemistry" 

(trans,  by  Dr.  M.  H.  Fischer).     1917. 
Wo.  OSTWALD,  "An  Introduction  to  Theoretical  and  Applied 

Chemistry"  (trans,  by  Dr.  H.  M.  Fischer).     1916. 
ZSIGMONDY,  "Colloids  and  the  Ultramicroscope "  (trans,  by 

J.  Alexander).     1909. 
ZSIGMONDY,  "Chemistry  of  Colloids"  (trans,  by  E.  Spear). 

1917. 

FRENCH 

COTTON  ET  MOUTON,  "Les  Ultramicroscopes  et  les  objets 

Ultramicroscopiques."     1906. 
PAUL  GASTOU,  "  L'Ultramicroscope  dans  le  Diagnostic  Cli- 

nique  et  les  Recherches  de  Laboratoire."     1916. 
PERRIN,  Numerous  Journal  Articles. 

85 


86  BIBLIOGRAPHY 

GERMAN 

ARTHUR  IV!ULLER,  "Allgemeine  Chemie  der  Kolloide."    1907. 
Wo.  OSTWALD,  "Grundriss  der  Kolloidchemie."     1909. 
THE  SVEDBERG,  "Herstellung  Kolloider  Losungen."     1909. 
FREUNDLICH,  "Kapillarchemie."     1909. 
VAN  BEMMELEN,  "Die  Absorption."    1911. 

The  "Zeitschrift  fur  Chemie  und  Industrie  der  Kolloide 
(Kolloid-Zeitschrift)"  and  "  Kolloidchemische  Beihefte," 
published  by  Wo.  Ostwald,  are  mines  of  information,  con- 
taining both  original  articles  and  references. 

Abstracts  of,  or  references  to  practically  all  current  articles 
and  books  on  Colloid  Chemistry  are  to  be  found  under  the 
division  "Physical  Chemistry  "of  "Chemical  Abstracts," 
published  by  the  American  Chemical  Society.  Furthermore, 
in  the  books  above  referred  to,  especially  Burton,  are  to  be 
found  numerous  valuable  references. 


AUTHOR  INDEX 


ALEXANDER,  J.,  50,  57,  85. 
ARRHENIUS,  S.,  38. 
ATTERBERG,  41,  44. 
BAHNTJE,  47. 
BAYLISS,  76. 
BECHHOLD,  26,  81,  85. 
BEHRE,  16. 
BEILBY,  49. 
BENEDICKS,  49. 
BETTS,  A.,  47. 

BlLTZ,   16. 

BREDIG,  10. 
BULLOWA,  57,  81,  85. 

BUNSEN,  16. 

BURTON,  33,  85. 
CAMERON,  45. 
COEHN,  30. 
COTTON,  23,  85. 
COTTRELL,  33. 

CUSHMAN,  46. 

DE  BRUYN,  LOBRY,  8. 
DUCLAUX,  69. 
EHRENBERG,  44. 

FlCKENDAY,  46. 

FISCHER,  M.  F.,  85. 
FREUNDLICH,  H.,  86. 
GAIDUKOV,  51. 
GASTOU,  P.,  85. 
GRAHAM,  T.,  1,  25,  33. 
HAMPTON,  69. 
HARDY,  10,  31,  32. 
HERSEY,  49. 
HUBBARD,  46. 
IGNATOWSKI,  23. 
JACKSON,  H.,  53. 
KEPPELER,  46. 
LANGMUIR,  48. 
LEVITES,  54. 


87 


LEWKOWITSCH,  52. 
LIVINGSTON,  45. 
LODGE,  O.,  33. 

LUMIERE,  45. 

MAXWELL,  38. 
MAYER,  54. 
MERKLEN,  52. 
MOOERS,  69. 
MOUTON,  23,  85. 
MUELLER,  25,  47,  85. 
NEWCOMB,  SIMON,  38. 
OSTWALD,  WOLFGANG,  10, 14, 

85,  86. 
PATTEN,  44. 
PERRIN,  85. 
PIERONI,  14. 
PUTZ,  49. 
RAFFO,  M.,  14. 
RAMSEY,  8. 

RICHARDSON,  W.  D.,  54. 
SCHAEFFER,  54. 
SCHREINER,  45. 
SEYEWETZ,  45. 

SlEDENTQPF,  11,  19,  23. 

SPANGENBERG,  46. 
SPEAR,  E.,  85. 
SPRING,  W.,  14. 
SVEDBERG,  THE,  86. 
TERROINE,  54. 
VAN  BEMMELEN,  86. 
VAN  CALCAR,  8. 
VON  WEIMARN,  P.  P.,  10. 
WAGGAMAN,  44. 
WUST,  49. 
YOSHIMOTO,  68. 
ZSIGMONDY,  7,  10,  11,  16,  17, 
25,  48,  55,  85. 


SUBJECT  INDEX 


Absorption,  19,  86. 

Acidosis,  73. 

Adsorption,  16,  50,  82. 

Agriculture,  43. 

Amicrons,  22. 

Astronomy,  36. 

Atmosphere,  40. 

Boiler  scale,  65. 

Brewing,  61. 

Brownian  motion,  8. 

Cardioid  Condenser,  23. 

Cement,  66. 

Ceramics,  42. 

Chemical  Analysis,  68. 

Clay,  42. 

Coehn's  law,  30. 

Colloid  Chemistry,  Applica- 
tions of,  36. 
definition  of,  6. 

Colloids,  Classification  of,  10. 

Colloidal  protection,  24,  78. 

Comets,  37. 

Confectionary,  61. 

Cosmic  dust,  37. 

Deflocculation,  35. 

Dialysis,  25. 

Diffusion,  25,  27. 

Digestion,  74. 

Dimensions  of  colloidal  par- 
ticles, 7.  22. 


89 


Dispersoids,  10. 

Dyeing,  50. 

Edema,  73. 

Electric   charge  of   colloidal 

particles,  30. 
Electrophoresis,  30. 
Electroplating,  47. 
Emulsoids,  12. 
Enzymes,  75,  78. 
Excretion,  79. 
Fatty  Degeneration,  85. 
Faraday-Tyndalleffect,20,39. 
Filtration,  66. 
Foods,  70. 
Geology,  41. 
Glaucoma,  73. 
Gold  number,  32. 
Hydrophile  colloids,  12. 
Hydrophobe  colloids,  12. 
Ice  cream,  60. 
Irreversible  colloids,  10,  12. 
Isoelectric  point,  32. 
Kidney,  81. 
Lyophile  colloids,  12. 
Lyophobe  colloids,  12. 
Metallurgy,  48. 
Meteorology,  39. 

Micron,  (M)  =  YQQQ  mm< 
Migration  of  Colloids,  31. 


90  SUBJECT  INDEX 

Milk,  56.  Radius  of  molecular  attrac- 

,     N          1  tion,  14. 

Millimicron,  M  =  1^0  *        Reversible  coUoids,  10. 

1  Rubber,  64. 

~  1,000,000  m  Schutz  kolloide,  44. 

Mineralogy,  41.  Secretion,  79. 

Mortar,  66.  Soap,  52. 

Nephritis,  73,  85.  Soils,  43. 

Pathology,  71.  Solution,  7. 

Pectization,  31.  Submicrons,  22. 

Pedesis,  8,  53.  Suspensions,  7. 

Peptization,  31,  33.  Tanning,  63. 

Pharmacy,  69.  Tyndall  effect,  20. 

Photography,  65.  Ultrafiltration,  25. 

Physiology,  71.  Ultramicrons,  22. 

Plaster,  66.  Ultramicroscope,  17. 

Purple  of  Cassius,  16-  Weather,  39. 


LITERATURE    OF  THE 
CHEMICAL    INDUSTRIES 


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TABLE  II 


LjO. 


Suspensio  n-s- 


uold  Sus- 
pension G 


Classification  of  Colloidal  Solutions 

according  to  the  size  of  the  particles  contained  in  them  and 
according  to  their  behavior  upon  desiccation. 


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UNIVERSITY  OF  CALIFORNIA  UBRARY 


